The way i do it in the market is this:
I always look for the highest price player, then for His age, If a 21 years old have a higher price that all the youngester players on the market he is the player i whant, not the 18 years old players, because If he is older, but have a higher MV he trains fast. This If they have same number of stars.
If there are any high MV young players, i just look for the highest quality players no matter the age.
A 27 years old player can stay in your team 3 seasons without any problem.
In both ways i only buy If i need the position the player have.
I dont look for quality whend selling a player, but to ratings and stats, in my 15 seasons at this game i have seen that even some 1* players can be better that some 6* players.