Also it's so easy to end up with seller's remorse.
You sell a young player for a fee that doesn't just reflect current ability but potential ability to. You visualise a line on a graph and think, "OK, so a mid-career player who can do X and Y now would be worth £30m, but he's young and on the upswing, that's gonna bump it to £40m, £45m, maybe more."
If you agree to the sale and the player does actually follow that trajectory and fulfill that promise then you're going to end up at best feeling wistful for what might have been, and at worst feeling like you got mugged. Jota's one of those.
But you could also turn the money down and then realise your rising star was merely going through a purple patch, and you've mugged yourself. Adama Traore was one of those.
Half the time - turning down the sale and watching your player develop as hoped, or making the sale and watching them fall flat on their face while you laugh to the bank - you're going to be happy, but half the time you're going to be very disappointed.