Simple fix ... Every account only gets 3 drops of any item. If you trade away all 3 vex or galleys etc you will not receive any more. Also if an item is traded (sold) more than once, every 3rd time it's transacted you have to take it to the gunsmith or xur to be repaired. This would add trading and limit the underground plus require the 3rd hand item to be re-leveled (repaired). The repair process would require some work pertaining to the origin of the weapon, a number of strikes, or some raid completion in the case of raid rewarded weapons.