You should create a new Gmail account for each game. Add those Gmail accounts to your phone, then link your games to these new Gmail accounts.
When you sell the game, you can then easily give the buyer the GMail account and transfer the game data over if there's some transfer code system. All games are different.
Making a new Gmail account isn't required for all mobile games, it depends on the game and how the transfer system works, but that's the easier way imo.