Sorry. But in this universe, only Jawas may sell used droids and speeders, as well as other used junk they find. You must keep all of your used stuff. You may only sell new unused speeders and armors. If you use it, you must keep it, or you can sell it for a substantial loss to an NPC, but not to another player. Except for speeders, you cannot sell a used speeder to NPC's, either.