I would say traders that belong to factions could have bonus on their profits in the same way miners with all mining stuff have mining bonus.
Another thing is contraband made by independent traders and affiliates: currently they are rarely sanctioned by polices. Thus we have a lot of people who simply don't mind about trade rules (I've seen a lot of unauthorized people transporting plutonium or Ageira Tech stuff). I guess we could imagine the jumpgates are scanning cargo of all ships using it. Then jumpgate could send scanned data to local authorities that could fine smuggler. To represent that fact, I would suggest that everybody would be allowed to scan shippers and report smugglers in an oorp forum. Then appropriate sanction could be taken. The idea is to make smuggling more risky and to incite people to join trade factions in order to be allowed to transport profitable goods.
(Note that I've supposed members of trade faction does not smuggle)
