Negotiation players are *automatically* put on the list when they are near their next star, 5* to your perspective. You don't add your players the game does. The list shown sometimes has players you cannot buy on it, too. You can negotiate for players from friends & others on your server, you can do this for players 3-5* from your perspective/level view.
You can only buy 6* players directly from Mr. Scout.
You can sell/buy your players on the tranny list as well. They will be 3-5*.