Limited time offers with unnecessary urgency.
One psychological trick that pushy salesmen use all around the world is artificial scarcity, and they often combine this with an unnecessary demand of urgency on the buyer. This is why you often see items listed for sale saying something like "Only 4 left in stock. Order now!"
Game developers use this technique to trick you into spending more time and money on their games. This often takes the form of limited time offers. The only reason for the short time limit is to make you rush into making an uninformed decision and prevent you from comparing the value of different items.
Games may also advertise items by saying that there are only a certain number available, "Hurry, only 47 magic daggers left in our inventory!" In a game where all the items are digital, there is no reason to artificially limit the supply of an item. In fact, often these numbers aren't even real and they may sell far more than the stated limit. The only reason for emphasizing the scarcity of the item is to make you hurry up and make a purchase before you think better.