Players are only worth what someone is prepared to pay - just like your car. The site is owned by Axel Springer a German publisher and covers all football leagues in Europe and maybe more. So for comparative data it’s unrivalled. I don’t know how they do it, but Tarks got a 100% hike in his value. Vardy and a few Leicester players were downgraded. I suggest that players in demand will sell for 2-3 times their value. Players value will then adjust to reflect the new sale price.
See Michael Kane here. We got £25m when his TM value was £10m. His Everton value is now £20m
https://www.transfermarkt.co.uk/mich...spieler/118534
Here’s Andre Gray https://www.transfermarkt.co.uk/mich...spieler/118534