Hi Levant and welcome in the forum
I merged those two threads as they have the same subject.
The value of a player hasn't anything to do with his performing or quality value.
Often we use it as an indicator if this player is a fast trainer (has a quicker development).

I want to take advantage and I want to ask you when is the best moment to buy and sell players, when the season has started or at the end of the season.
as Bunzo said, now it doesn't matter as a bot manager is buying our players.
Best time I think is in the end of the season as the new one they will be one year older and they 're gonna loose some percent of their value (specially if they are young and expensive).
But are you manage to find replacements in the first day of the season ?
Transfer market is little crazy now.