depends if i need more cash, then i sell thru auctions. if i need tokens, then i sell thru nego with tokens. what i meant to say above is that i'm only selling players that i really want to sell. in a tough server, i will not be enticed to sell any of the players that i need even if he spends 50 tokens and i get say half (i'm not sure if this is correct) of that. for 25 tokens, i don't have assurance i will get the same type of player or better.
you can build up the players you are selling such that only one point is required to go to scout level to your intended buyer. having multiple positions and skill is a bonus. aside from this, there is really nothing much you can do as a seller thru nego. if this type of players don't cause bidding wars (more than 20 tokens) in your server, i think it is safe to say you will rarely get any nego offer which will cost the buyer more than he will spend thru auction. it's a good idea to observe the auction for high end 3-4 star players in your server to get a feel on how active are the managers below you.