i dont remember if the players loose their market value when u go next stage???
Also with my small experience from the game ,i believe that to take a 5* player which is close to next star before the end of the season,is a good choice to replace a sub player with small quality in your squad.
For example i have 2 MCs 23-24 level with 60-70 skill points to go for the next *,i bought 2 news 29 level with 7-10 skill points to next * even if i know that they will be minus 1 star in a few days,and i will sell after my 2 finals the 23-24 level players