Buying players isn't correct legal terminology. Buying humans is illegal under human rights law. What clubs do is in fact paying compensation to the players club for breaking the players contract. So EU trade-law doesn't apply.
This is also why players can free themselves from their contract when their club is demanding a transferprice deemed unreasonable. Player will just have to repay the club for any damages suffered (usually considerably lower than any transferprice). It's because off this risk that clubs started using minimum release fee clauses: if a player knows in advance what the minimum release fee is he can't argue that it's unreasonable.
