I've been wanting to do this for the longest time but somehow never get around to it so am just going to take the time to do it.
Bear with me, please, because this will be a bit lengthy, but if you're interested in making more money in the MP you'll want to read it.
First of all, I am by no means among the elite rich people here on Gaia. I do not support myself via Gaia Cash, I support myself through the Marketplace. I know there are others out there who are more knowledgeable than I am but I've definitely learned a thing or three in my time on the MP, the kind of things that would profit anyone to know. When I do buy something with Gaia Cash it's for myself, not for making profit. When I get something like a random gift box I always open them. Most of the time the item is garbage but if it's worth something and I don't want it I toss it into the Dumpster (via the Donate button in my inventory) because really, who doesn't love to pull something halfway decent out of there? I do have a couple of exceptions. I sell Philosopher's Caches if I'm lucky enough to get them and sold one Golden Magical Giftbox because I was on my last leg of being able to afford something REALLY expensive.
I'm not going to tell you exactly how I earn my money (there are many ways in which people can do really well) but I am going to outline some general tips that will help you make more profit with your Marketplace selling.
1. Expand Your Store: If you've sold over 1,000,000 gold worth in the MP make sure to follow the Achievement chain that will unlock 20 slots in your store. You don't necessarily have to fill those slots all the time but they come in handy when you need them.
2. List In Bulk: If you're listing game items (ie: flowers, trash, bugs, fish, etc.) unless it's a super expensive item like the Stardust that will occasionally come out of a fish drop, and you have multiple items LIST THEM IN BULK!! I cannot express how important this is. Your items will sell much faster and at a higher price than single items listed! Buyers find the larger listings by clicking on the Unit Price. Most of the time when I'm looking to buy I have my page showing the maximum, 100 listings, to make my buying go faster.
3. Learn Who Your Competition Is: If you're selling 100 Crumpled Newspapers, for example, you are NOT competing with the seller with 1 or 10 or even 50. You are competing with the seller who has close to the amount you do. Set your per each price at slightly higher than the ABP. It may take a little longer to sell but it will sell.
4. Haste Makes Waste: I see it a lot. Someone is in a hurry to sell so they undercut everything else by a lot just to be first in line. Depending on what you're selling, it could be worth it. Most of the time it's not. Use the Graphs to see how many of that item sell in a day. If it's a fast-moving item, a higher price will be met within a reasonable amount of time. If it's a slow-moving item sometimes it's best to take a lower price. Common store items are usually the exception to this rule and that's just because certain of them don't go across the MP frequently enough to see a lot of sales. With these items maybe one is listed every few days and sells off pretty quickly.
5. Know What To List and What Not To List: I see it constantly! I click to buy someone's stuff and of course I look through their whole store. In addition to two or three items worth a reasonable amount of money their store is filled with garbage. I'm not talking about town or fishing trash--that's garbage worth selling. I'm talking about all of those aquarium and other items that come out of the Daily Chance and Dumpster Dive and fish drops every single day of the world. I'm talking about the ones that most of us get multiples of every single week and sometimes every single day and get listed en masse in the MP for 1 gold each. Seriously, people, rarely does anyone buy that stuff, and your store slots are WORTH SO MUCH MORE!
It takes what, five minutes to list a store full of 1G garbage that very few people want to buy? And of course the reason so few want to buy it is because they're pulling the same stuff out of DC and Dumpster every day too! In just fifteen minutes at Bass'Ken Lake you can fish out an entire bucket full of the easiest fish there are plus a few odds and ends of trash. Even with half a bucket of trash it's worth at least 10,000 gold. Plus, you've just put the fish bait you get out of the Daily Chance to good use. In fifteen minutes you can play at least thirty hands of Black Jack and usually make at least half of the tokens bet back in tickets. That's roughly a 5:1 pay-out, and while it isn't a lot of money the tickets sell on average for 10 gold each in the MP. In fifteen minutes you can run through at least a half-dozen towns, maybe more, and grab all the pickables. Again, worth WAY more than the items that get listed for 1G.
Every day I go through my Aquarium items, select them, and choose "trash" on several backgrounds and aquarium ornaments. I do not put them in the Dumpster because no one else wants them either! They absolutely are not worth my time or inventory space and they sure as heck aren't worth one of my precious 20 store slots.
6. Common Store Items: If you get something out of the DC or Dumpster that is an item that is commonly sold in stores and you're thinking about listing it follow this guide:
I'm not talking about Premium items here, ONLY Common Store Items. In your inventory, click on the item. Compare the Sellback Value to the Average Buy Price in the MP. Is the Sellback Value higher or lower than the ABP? If Sellback Value is HIGHER than the ABP, choose "Sell Item" right then and there. If the Sellback Value is LOWER than the ABP put it up for sale in the Marketplace at a reasonably discounted price from the store price (I'm talking about 75% to 80% of store price, not horribly lower but enough to give a potential buyer an incentive). Every day there are countless equippable and aquarium items listed at prices lower than the Sellback Value. Other people buy those items and sell them back to the stores (shoot, sometimes I even do it if I happen to catch one) and you've just lost profit. My most favorite items I get like that are some of the large stickers for the aquariums. They have a store price of 5,000 gold which means an instant 2,500 gold added to my coffers.
This by no means covers everything but should get you started. I'll re-read later and most likely amend this.