There are many ways
1) hit up Dumpster Dive whenever you are able (you can dive in every 5 minutes). You won't get "good" things all the time, but you can on occasion. The cap was raised on the value of donated items, so you can get things valued in the billions (not sure what the cap is, but I did get an item worth 3 billion two or three weeks ago).
Most of what you get will be alchemy cache's. You can either open the Philosopher's cache's to get the colored ones (which have no sellback value) or sell the Philosopher's cache right up for 25k each. You can also get heart charms from them, but it seems that when Gaia added zOMG cache's to pile the odds at which you get heart charms was dropped dramatically. I opened like 70 cache's and got 1 heart charm.
The colored cache's have ingredients in them, some are worth 1 million or more gold a piece. The annoyance with this is you can only open one cache at a time. But it's a free way to wrack up gold.
2) Towns (the original) - Play during Dusk and Dawn, collect everything you can find and then sell it in the MP. The best Towns trash is Old Crumpled Newspaper, which can really only be found during dawn / dusk. It's currently selling for 2.5 million gold a piece. Back when I played Towns I found I could average 20-30 pieces per dawn/dusk cycle. Another words, 20 minutes of game play can wrack you 50-75 million, JUST from old crumpled newspaper. They you have the rest of the junk to sell too (most, I will admit, aren't worth much, but it's still gold).
Night is the next best time to play because of blue carnations, but Dawn/Dusk, which occurs once every hour, is certainly the best.
3) If you just want to chill out and play a game and make gold doing so - Blackjack. It's been a while since I last played, but I seem to recall one full circuit in that game (playing all 3 NPCs) lasts about 45 minutes, depending on how rapidly you play each hand, and I could come out with around 1k tickets (give or take some). At the current going price of tickets, 1k tickets would net you around 70 million.
4) be sure to hit up the Daily Chance carts every day. You get free gold AND GoFushion charms - heart, stars and even diamonds. Sell them off for gold in the MP. Tidy profit for about 30 seconds of work.
5) Vending is a traditional method - buy low, sell high. The issue with this is you have to know how the MP works. Not only do you have to check the average buy cost, but you have to check the charts to make sure the items was artificially inflated.
The other issue with it is that Gaia is constantly re-releasing items. So an item you buy today could drop in value tomorrow because of a re-release.