Trading is one of the most complex system to implement in any game because the second you introduce it, a market is formed and a quantifiable value is placed on the items. This demand can translate into real-world currency. You'd also get complaints of scams and this game becomes prone to online markets and hacks because real-world value can be place on items. The market would hyper-inflate value of highly sought weapons: Ghally, IB etc. while other exotics hyper-deflate into legendary status. This trading system would also immediately have an effect on Xur and strange coins. People would also then just farm commendation for legends and try to trade up for exotics. All of these problem +more happens as soon as trading is allowed. Bungie, a company that can't even fix the raid, will not be able to handle such endeavor at this time. GG