I don't like player to player trades but I do think you should be able to dismantle or otherwise sell something you have for your choice of what you want in return. Everything can be attributed a value in terms of some other item. This could be a difficult thing to manage. But I would think some common scale that equates the relative value of items on it would do. Maybe one planetary mat is 1, a weapon is 50, a core is 500. Then anything can be traded for something else. You could trade a weapon for 50 mats or you'd need 10 weapons to get a core. Or you can trade 500 mats for a core. Or a core for 500 mats. It's all a matter of setting the relative values. There should also be a cost to each trade to attempt to keep players from finding some path to trading up or otherwise gain the system which isn't the intention.