There are no agent fees at the moment. In the future, there will be, and users will be penalized slightly for selling a player they have just bought (to make good day-trading a bit tougher).
At the moment, you are allowed to bid on your own player. We'll figure out how this will affect transfer fees when they are instated, but I don't see a reason offhand that we shouldn't allow users to "bid up" their listed players.