Limits should exist and players should cost a min you're right. Just higher the min and don't take an extra token from the buyer for every token he wants to give the seller. It's as simple as that and everyone would be happy. When a 18yo in negotiations would cost 18 instead of 10 to buy in and every single token you offer goes 1:1 to the seller, this system would be much better and you would be able to sell the players you once liked for decent prices.