Often the only player I can sell to lower level friends is the free Academy youth, due to the level difference.
To be clear (the stars Q is for the buyers' level, not the sellers):
Auctions is players 3-5*
Negotiations is all 5* about to be 6* (you have to be level 4 or above and using desktop version of game)
You can make an offer on any player on your server from Teams of: Friends, Cup, Champs League & League that is 3*, 4*, or 5*
Scout is 6* (and if you are in a higher than your own level League 7*)