Yes but Gert Funck told you something very important there as well Altiplano so if you want to increase your chances on selling him follow the advice.
If he would be fresh 6 stars the others would see him as a fresh 3,4 or 5 stars so would be quite hard for you to sell him after that. Better you can invest the skill points into abilities or positions (roles).

Basically on auction you can find from 3 to 5 stars, on negotiations 5 stars and on scout 6 stars but in special conditions you can even find 7 stars players.

PS: On special cases on negotiations you can find players who are 3 or 4 stars as well but this is something very rare. It occurs only when somebody do an offer for a player from one of your friends teams.