Price Fixing (Pump and Dump)
This is where you have one person, or a bunch of people, collude together to artificially inflate prices. They will buy an item at 2,000 points, maybe even 7,000 points, whatever. They buy tons of them. Then they post messages in chat rooms and post in the trading post about this item rising in price.
The price actually is rising, due to the artificial demand. (This is called pumping). I saw this happen with liquid goo. It literally went from 6,000 points (when I saw it, they claimed it was at 500 the day before, which may or may not be true) to practically unbuyable, then falling back down to about 10,000. They just posted over and over that the price was going up, and that it would soon be unbuyable. This was true, because everyone was buying it up. It was a self-fulfilling prophecy.
Then they get their items that they had previously bought, and sell them off while people are still buying. (This is called dumping). You can even get in on it, if you do it early enough, even if you weren't involved in the original plan. If you do want to do it, just post in the trading post or chat boards that it's becoming unbuyable, prices are going up and up and up (and they are) and you should buy it NOW. You can make a killing. Not sure if it's against the rules though.
People do this with real stocks, buy up a bunch, create demand, then sell it while the price is rising. It's illegal in the real stock market, but man . . . you can make tons of money.
To pull this off, the item you want to use has to be in the range of 5,000 to 10,000, and if you were to buy up 20 of them in the grouping, the price would double. You can't do it with things that are as abundant as codestones or scratchcards, cause you would need too many people to buy too many items. It would never rise that fast.
What you need is a scarce item, and if you were to get other people to buy up 100 or 200 of them, you would double your money. Those are the items you should use. Also, items that no one ever heard of either, that helps. If people know what it is, and know the "real" value, it's not going to work.