There would need to be restrictions as to what could be traded. normal items could be traded, ascendant materials would need to be exchanged for something equivalent, same for exotics and legendary items. Raid legendary's would require proof that you've beaten the raid on the difficulty needed or else you won't be eligible to recieve it. I.e. Atheon's Epilogue requires you beat normal before trading, fate bringer requires you beat hard, etc. If there would be a trading system, thats about how it'd have to work.