Well most of us use in game money to do that, so I'm not exactly sure what your thinking is sorry. Maybe you can go play Dub's game *which will probably never get done completely* Considering SCWN was down about 70% of the time lol..IDK other games. But I'm sure you and Dub would make best of friends.
Stargate Wars, old browser game. Players can trade population units.
Buying or selling pops is one of the semi-new things to SC i find interesting. But with the introduction of GC its seems wise. GM has been sure to make selling nation suffer.