From the game manual:
When selling a player, some of the purchase price is used to pay agent fees and other taxes on the sale. The fees and taxes for a sale are determined by two factors: the amount of time that the player has been on your team, and the total number of sales your team has made in the previous 14 weeks. The fees are lowest when you have owned the player for a long time, and when you have made few sales in the preceeding weeks. Each of your players will have the currently-applicable fee shown on their individual player page.
http://www.buzzerbeater.com/community/rules.aspx?nav=TransferList