Exotic Salesman

This can be done from the trade menu, openable from the top-right quadrant of your screen.

You'll need to have discovered a faction to trade them with to be able to do this - so if its still showing unavailable for you then explore more to discover the other minor factions.

This is soft-missable because if the factions you can trade with are destroyed then you'll no longer be able to sell any in the match.