How about not add a trading system? If you do make it be Same year, same armor alot, same rarity (like exotics with exotics), no gun trading, and same power (like an exotic 290 for another exotic of 290). That way people just dont give out their duplicates to their friends. It would create a disadvantage for those who don't have people to play with and takes away the grinding for those who want the easy way. People would also "sell" their exotics for real world cash.