Player Value question

I had always thought that player value was tied directly to age and how fast they train but I think I had a situation that proves otherwise. That is if you have two players of same age the one with more value trains faster. however, I sold a player who was maybe around value $2.5M and he sold for $3.7M. Then when I looked at his new market value it was 3.5M. So this means that while value may be linked to age/training speed at some point, if there is a bidding war on a player his market value will increase considerably.
so you really can't trust market values to tell you much. If a player is at 6M and all similar rated players are at $3-4M , this only means the prior owner overpaid. Has anyone else run across this? or come to this conclusion.