@rado-kochas
Hi there,
Only the developers knows all ins and outs of the transfer system in OSM, but it is suffice to say that it is pretty complex and that there are a number of things that will affect it. For instance: how expensive is the player and for what margin are you selling that player for? Normally speaking: the more expensive the player is, the more difficult it will be to sell him.
But even if you try different things, there is also a huge randomness in play. So it's impossible to really predict what works and what doesn't. Sometimes you sell an expensive player really fast and other times you have to wait for some time, even when you play with the margins.