Granted, everyone listens but others make better decisions over when to sell and when not to sell. The idea that every smaller club is duty bound to sell whenever a bigger club comes knocking is a myth. Some clubs - E.g. WBA with Berahino and Spurs; Southampton with Wanyama and Man U/Arsenal; Everton with Stones and Chelsea - know how to say "No!" Some make an assessment on whether selling a player is in their long term best interests and ensure they get above market value when they do sell, E.g. Newcastle, £35m for Andy Carroll; Southampton £60m combined for Shaw, Lallana and Lambert in 2014 close season. Maybe Mansfield are employing a similar approach with Ryan Tafazolli and The Blades?
Getting proper value - which we often don't (Jags £4m, Walker £3m, Maguire £2m) - softens the blow and provides the opportunity to rebuild, as Southampton proved in buying Pelle, Bertrand and Tadic.
Our approach seems consistently short-term and more preoccupied with the player's best interests than the club's, i.e. "if a player has stated his desire to leave it's in everyone's best interests to sell." Well selling Blackman, McDonald, Maguire and Murphy without adequately replacing any of them has not furthered our cause has it?
So, yes, most clubs will sell if the price is right, but the successful ones reinvest with the expectation of improving the team.