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.