The prices are going crazy with all these people buying players and selling them immediatly after with an higher price.
If you look at some transfer history pages, you can see that some players bought and sold up to 100 players in a season time. It's really unrealistic to see this...
I think it should be forbidden to sell a player bought within, let's say, 3 weeks (most of the daytraders sell their players within a week), and the fees for re-selling the player too quickly should be higher. For example, it could be 50% if you sell him within a week, and then 5% less per week.