I'm going to use FIFA12 Ultimate Team players as examples in this post, but the theory can be applied to most any game with an in game market and verying degrees of rarity and availability of items to trade.
In order to make easy money you have to identify a commodity which, whilst abundant at the moment, will become rare at a later date. This is easy with Ultimate Team as each week players who are performing well in real life will have an "In Form" version of their card produced which is available in packs for the duration of the week. What this means is that this in form card will probably be filling the marketplace for the week and the couple of weeks following, but supplies will dwindle once it is no longer available.
|In-form Louic Remy|
So, your job should be to get your hands on the commodity whilst it is in abundance. The price should be low because, no matter at what price a player is willing to sell a card, there will always be another player willing to sell lower. So, once you feel the card is at its very lowest price, that's when to buy. With any other item (one available all the time), this process would continue ad infinitum until the game ended; as more and more players pull them out of packs and seek to sell them on quickly, driving down the price. Even the rarest of item's prices will be driven down inexorably if production is steady (and could even drop as low as just above the price at which you could discard/sell the item to an NPC merchant).
Once the week is over, hold onto the card for a while. Supplies will still be high, and it is likely that players who found the card in packs will probably seek to sell him on once they've tried him out for a few days. After a week or so after the production of the cards stops, the supplies in the marketplace will begin to fall as some players discard him, stick him in their club and forget about him or the account he is on is either hacked, banned, deleted or simply abandoned and unused. As the supply drops, the price will rise. Then; sell sell sell.
This is a theory which can be applied to almost every game, there are, however, caveats to remember:
1) You have to be careful about buying rare commodities which could be substituted for common alternatives with little downside (or even and upside).
2) Rare commodities' prices are subject to the degree of availability. Hence why ectoplasm prices plummeted when the shadow form assassin build was an easy way to farm for them. Now that the process is much harder, the prices have risen again. Any limited availability item you buy will be subject to the same rules, if the items should suddenly become available again, well, you're out of luck and are likely to not quite make as much dough as you thought.
3) Sometimes you will lose, you will misinterpret where the market is going or the developers will change something which renders your well laid out plans totally pointless. It's annoying, but it happens.
Other examples include: festival items such as mini celestials from the dragon festival, max damage req7 swords and axes, runescape's event items such the Party Hats. All things which have/had limited availability and are now worth a small fortune.