When I buy players off the open market I only go for the ones that are very close to the next star - regardless of what positions, age or abilities they have.

A player who has just become a 6 star player may look good to you at level 20, but to a player in level 21 that same player is a poor 5 star player.

I wouldn't be interested in buying a player who had only just become 5 star, so why would anyone else?