Transfermarkt value Issa Diop at £19.8 million. He's just moved for £22 million. Transfermarkt value Łukasz Fabianski at £5 million. He's just moved for £7 million.
Transfermarkt value Paddy McNair at £2.7 million. We're happy to accept £5 million for him.
There's absolute consistency there. Players are sold for a couple of million more than they are valued on Transfermarkt.
Transfermarkt valuations are given by members of public signed up to the site and generally runs out of the forum part of the site and is put to a moderator(anyone can apply to be a moderator or data scout). It's completely amateur and there's no expertise in it. Most of the time they're putting valuations in place without ever seeing the player as it's mostly german users. Anything which involves guesswork like valuations should be taken with a Wiki style pinch of salt. Not to say they never get it right. Just saying it's down to a non expert personal opinion just like anybody on here.
No need to get arsey. I was just a member when I wrote for RR so was able to share how it works.