I find odd that a youngster with market value of 50K euros, and sold for 250K euros keeps a market value of 50K.

I think german website transfermarkt.de adjusts the value of a player to -more or less- the last transfer price. After all, « market price » is the price the market would pay.

So maybe, after a transfer is closed, market value could/should be adjusted with a formula like : New market value = (50% old market value + 50% transfer value)

Same could work downwards with old players.

Hmm, interesting. Based on it, my player Istvanffy's new value is: old value/2 + transfer price/2 it is 250K+550K= 800k Eur.
http://rockingsoccer.com/hu/foci/info/player-153015 -he is the player of mine.