Transfer fees reflect the value of a contract and so, agreeing a new one, pushes the price up and it might deter suitors, but I don't think we'd have any difficulty selling Gueye if we wanted to.
I think you're neglecting the other side of a player's contract: the player's wages.
There are some frankly outrageous salaries being rumoured for the likes of Sandro, Sigurdsson and Klaassen. It wouldn't be at all unreasonable for him to demand parity with them if that was true.
Everton might not have a problem finding a buyer, but Gueye would almost certainly have trouble finding somebody who'll match a salary of £100k+/week for that length of time.