For each item we've included, we've included some info on why we think it'll be a good investment, the times we think it'll be the best to buy/sell, as well as a few words of caution and anything else you need to know. If you have any questions about any of the items on this list then just post in this thread or in the main forum; we'll gladly tell you what you want to know.
Of course, sometimes our senses aren't perfect. We do occasionally pick investments that don't really do as well as we've predicted. We can't guarantee your money, so invest at your own risk. But don't worry, our bad eggs should be far and few; investing in more than one item in this list should guarantee that you don't lose everything.
This list isn't exhaustive; there are plenty of other good investments in the market should you find them. This thread just lists the items the crew considers the the better investments available. Also, what we've listed as best times to buy and sell are not 100% set in stone. You can hold on to your item for as long or as short as you want; these time frames are only our minimum recommendations.
With all that said, enjoy the list! Just click on the pictures to go straight to the Marketplace listing of any item. ^__^
Notes about the Market:
As we're well into December, we'll probably start to see prices on many items climbing very soon. Now is probably your last chance to stock up on any bargain deals before prices start to move uphill.
Also, if you still have any Chance Items from 2010, now is a good time to start dumping them. The Super Prize CI had a severe (albeit temporary) impact on prices last year, and if it's released again this year we'll most likely see the same pattern. Selling now (and, if you want to, buying back when the prices DO drop) is highly recommended.
Mid- to Long-Term Investments
(6 months or more)
(6 months or more)

Item info The Muramasa, a long awaited cheaper substitute for the classic Ancient Katana MC. This item has a ton of potential. It's unlikely that the Muramasa will take any hits that will cause it to deflate but just in case, it is recommended that you hoard the letter over the actual item.
Best time to buy This item is going to be a little tricky to buy because of it's popularity. Watch MP graphs closely for signs of dips. It will likely drop in price whenever a new RIG is released or evolving day.
Other info Because there hasn't been a strong MC for a while, the Muramasa could be 'overhoarded', which may stop it from rising for a few months. And as with all MCs, the letter is preferable to item itself.

Item info One of 2010's strongest MC's so far. It's a classic item that will remain in high-demand because of its timeless poses, making it a solid long-term investment.
Best time to buy Before December
Best time to sell Valentine's day 2011, or whenever you're satisfied with your profit.
Other info The best times to buy and sell are based on the expectation of a Valentine's spike—red roses sell best around this time. As with all MCs, the letter is preferable to the item itself.

Item info The Solar Sash is likely the surprise of the recommendation list. We reckon it's very under-appreciated, and it's just a matter of time before people figure out it's true value.
Best time to buy Nothing specific, but as soon as possible is best. Wait for a new Cash Shop release before buying so you can pick a few up at a slightly cheaper price.
Best time to sell Whenever you're satisfied with your profit.
Other info As with all MCs, the letter is preferable to the item itself.

Item info One of the better MC's we've seen since the N.Sash/S.Laurels. As a cute while bunny, this item will be in season during winter as well as Easter, making it a good long-term investment.
Best time to buy Before December while the price is still low.
Best time to sell This is a winter/Easter themed item so its spikes will be mid-winter and mid-spring. Though the longer you hold it the better, of course.
Other info As with all MC's, the letter is preferable to the item itself.

Item info A long expected complement to the Angelic Sash and Nitemare Scarf of 2003, the NM Scarf has huge long-term potential. This is a great MC to add to your hoards but keep in mind that their progress (inflation wise) will be slow. We choose to recommend this item not because we thought that it was going to inflate quickly over the coming year but because it makes an extremely safe hoard. That means the chances of it tanking on you are nil.
Best time to buy Right now is the perfect time to buy these. They've been stuck at 200k for quite awhile and have recently dipped below the 170k mark.
Best time to sell Because of the NM Sash's unique situation (i.e. the loong flat line) it's hard to suggest a specific time period. What we suggest to make the most of this investment is to take advantage of the constant price fluctuations—buying on bad days, selling on good ones.
Other info As with all MC's, the letter is preferable to the item itself. Be aware that this MC has an 'over-hoarded' tag hanging over it, meaning any time there is a small jump in its price inexperienced people will come along and dump causing the prices to drop again. It's only a good idea to invest in these if you have the patience to wait.

Item info Optimal purchase time is nearing for the SLaurels. These have recently taken a significant dip in prices now is the best time to get these before the Winter spike. These are still a terrific investment if played correctly.
Best time to buy Before December
Best time to sell Mid-December-February
Other info These have suffered a recent drop in demand, but we think this will be a short-lived rough patch come December. As with all MC's, the letter is preferable to the item itself.

Item info A well-made pair of slippers akin to the Panda Slippers. They will perform best in the Winter so it's still not too late to stock up on these!
Best time to buy Before December
Best time to sell Mid-December-February
Other info The Powdy Pengy was released in a Cash Shop bundle last year, so we are unsure whether they'll be released again this year or not.
One thing to note is another pair of slippers in the same bundle: Two Turtle Doves, which have proved themselves as investments just as strong as Powdy Pengy.

Item info A cheap alternative to the Kiki and Coco kitties this item is likely to bring in excellent returns over the next year or two.
Best time to buy Before the demand starts to spiral upwards. These items were already taken out of the cash shop so naturally the sooner the better.
Best time to sell As always the longer you hold onto an item the better but we think a spike is likely come December.
Other info Just remember that this item was sold in the cash shop and because of that it holds some degree of risk towards being released again. It also holds a risk of having more supply than the current demand can keep up with, which could mean it'll take a while to start rising.

Item info The bundle overall is a strong investment but the item that particularly caught our focus was the Punk Malice item. This item has great poses including an incredibly awesome hairstyle. It also fits in with a popular Gaia genre so it's sure do do well.
Best time to buy ASAP; the bundle has already started to rise in price, but keep an eye out for bargains when you see them.
Best time to sell Whenever you're satisfied with your profit.

Item info Despite the fact that this is an older MC the staff felt that if you have a little extra gold to burn and need something to invest in that this would be one of those items to target. Apart from NM/A. Minis, the Lusty is the top contender for fastest rising MC. It was an extremely under-appreciated item when it was first released. When people realized that Lusty layered great with these new skins people went crazy for it. Unfortunately by the time people realized the Lusty's true potential, it was too late, and it was long out of the cash shop. As a result there is VERY little supply for this item and an incredible demand.
Best time to buy Before November, Prices are likely to increase as we move closer to Christmas.
Best time to sellExpect to see this peak around Christmas this year and February next year. However, this item is likely to always to do well so if you're looking for more profit, you can wait for as long as you wish..

Item info This year's Halloween Cash Shop bundle. We're a bit late on this but we can say for sure that this bundle and anything inside it has great potential. The theme is great for Halloween, but also for non-seasonal avatars which means lasting demand. Another selling point of this item is its gambling factor and the rarity of the Spirit bundle.
Best time to buy The lowest point of these bundles has already passed and there's already been a price spike, so we believe the sooner you buy the better. Just keep an eye out on market trends and look for bargains.
Best time to sell Right now, we don't predict any seasonal spikes until next Halloween, if at all. We only recommend you sell when you're satisfied with your profit.
Other info Our favourite item is The Gravedigger but we believe that all these items (save the dogs) will do well. You may invest in the Requiem of the Living bundle if you want to but we don't recommend investing in the Spirit bundle alone, as we don't believe the risk is worth the reward.

Item info The bundle re-release of 2010's Cash Shop items. The prices of the items inside the bundle have plummeted because of this re-release. Out of all four bundles, this is the bundle with the most popular items—all these items have made great investments in the past, and there's good reason to believe they'll pick up again in the future.
Best time to buy ASAP. Prices are at their lowest right now, so the longer you wait the less you'll make.
Best time to sell The Unmentionables are great for Valentines, so that will be a good time to unload them. The rest don't have a best time, so just sell when you're satisfied with your profit
Other info Because of the chance at a Ballad Punk, the bundle itself is risky. However, the items (Malice Punk, Smok and Black Unmentionables) are very safe investments and we recommend just buying the items over the bundle.
High-Risk Investments
For you risk-takers, here are some current high-risk investments

Item info This is a zOMG! ring that is NOT soulbound. Based on the amazing performance of the similarly limited-edition Sweetheart Ring, this ring is a very promising investment and has already seen a huge increase in price. We expect to
Best time to buy ASAP; prime buying time has already passed but there are still good profits to be made on this ring.
Best time to sell Whenever you're satisfied with your profit. The price should spike when zOMG!'s epic items are released in the next couple months, making it an excellent time to sell.
Other info As a zOMG! ring, this investment comes with a fair level of risk: with the future of zOMG! being so unpredictable and so many ring balances, we can't be sure how the price will react. It's also entirely possible they'll put the ring up for sale in the cash shop or any number of things to allow people to get it. These events are unexpected, but buy with caution.

Item info One of the zOMG! event items, there are a lot of hopes for this item in the future. We expect the Cursed Pumkin to behave a lot like the Golden Egg from zOMG!'s Easter event which gave us the absolutely awesome Fortune's Favor ring. The reason this is something to watch out for is that it's a huge gamble. If we don't see anything worthwhile coming out of this bundle, chances are you'll be losing A LOT of gold.
Best time to buy As a rule of thumb the sooner the better. As the pumpkin starts to lose it's cursed seal, we reckon the price is going to rise higher and higher, which means less profit.
Best time to sell The Cursed Pumpkin will probably spike every time a little bit of the curse wears off, and then spike again when it's openable (if the items inside are good). You can either sell at these spikes or hold on for long-term profit.
Other info Keep an eye out for zOMG! announcements on this. Also, be prepared to lose gold; most of your gold is lying in the gamble that there'll be a new and useful ring in this, .
Low-cost Investments

Item info The clear winner of this year's Halloween event items, we expect this item to do pretty well in the long term. The main selling point of the Spectral Shroud is its cape theme. A similar Halloween item, the Ghost Cape was released in 2007 and has been reasonably successful. At such a cheap price, these should make a good low-risk hoard.
Best time to buy From winter through to summer, as long as the price stays low.
Best time to sell Next Halloween.