I don't think Bungie will ever do player to player trading based on what the lead developer has said but what about having a vendor that takes trade-ins. Maybe they are at the tower when Xur isn't. This vendor could buy back materials, ships, shaders and sparrows. I must have a dozen time breakers from VoG and discarding them gives you nothing. Considering a new rare sparrow goes for 175 marks getting 100 back for a gently used time breaker that was only driven through Omnigul once would be a great deal. As for weapons and armor, why not let the vendors that already sell them buy them back then make them available for others to purchase. Their current inventories are pretty shabby. They could have some fun with it like a exo in a trench coat selling ghallahorns for 50 motes of light.