very high transfer values of players - not according reality

Player valuations are determined by a number of criteria, but a big one is the players perceived importance to their current club. At the start of the season many of these players are effectively impossible to replace for their clubs. As the game progresses and they may fall out of favour, or develop issues with the manager/club these will change. 

Third party clubs can begin trying to destabilise the situation with enquiries/declaring interest so there are a lot of factors that will change this over time.