If we were buying everyone back then that is true, but likely it will be one in twenty or one in fifty that we buy back. The only one so far is Angelino and we sold him again at another profit. There is a link between the selling price and the buy-back price, something like 2 or 3 x the original selling price
If we sell 20 payers at £10M that makes us £200M orofit.
If we buy one back at £30M then we are still £170M up.
Plus, even if we don't want a player back but the buying club can sell them for more than the buy-back price we simply buy them and sell them to the 3rd party, or cut a deal to get a share of the extra £'s above the buy back price,