If you have a guy ready for negotiations (a tick away from 5 star for levels 4 and above) you should get a pop-up news when you get an offer; there will be an icon of a handshake next to his name in squad list. When you have the player card open, go to offers, and there it will be. You click on the offer, and can accept, reject or make a new amount as selling price. (If you are playing the portable version I don't think it works so well)
Transfers - Top Eleven Official Wiki