Bungie has explained before why there is and never will be a trading system, essentially everything you have in the game is a reflection of your own merit. If there was a trading/lending system how would you be able to tell who actually fought their way through that ridiculous Thorn Bounty and who simply bugged their friends to give it to them because the challenge was "too hard". Trust me I know the frustration of either wanting to help out your buddies or snag some of their outdated scraps, but nothing compares to that day when you fought your way to having that gear/weapon you always dreamed of!