If you bought (or enslaved) merc or slave (or even brahmin), it would be really cool to sell him to the other players. It could even grow up to the real slavers, which sell some slaves to lazy or are non-slavers (in a "slave hunters" meaning).
The option to sell would occur when follower is nearby and you stay 1 hex from a customer. It would be a traditional trade dialog like with merchants or players, but in your menu are icons of followers (skins). There would be no price set from followers. Just your customer need to deal the offer.
Of course, firstly follower bugs need to be fixed.