Buying a 3 stars players ending 1 at the end of the season, it is not the Best option, unless you really need one player for one particular position, and with 28 years old, he probably will gaine about 1 SP/day.
Since you are going up to level 6 at the beggining of next season is market value will reduce 10%, but since he was a free agent you can still make money.
Players 31 years old or more do not reduce there market value, even If you go to the next level.