But our fans would be saying sell him for 500k so the buying club is more likely to bid lower than £2m
Which in my hypothetical is less than the club is willing to accept, so the buying club would have to increase their offer. Maybe they'll hit 2m, maybe the increment they increase will jump over 2m. Negotiations between them finds a fee they are both happy with. Sometimes the buying team will pay even more than the selling team would accept, sometimes the selling team sell lower than the buying team was willing to pay. "worth what someone is willing to pay for" is a simplistic cliche, I agree, but 'worth what someone is willing to sell for' is equally simplistic.