Right now, if you want to trade an item that is bound to you, well, you can't... directly. You can, however, use it as a component to a more powerful item, and trade that item instead. That way, bound items aren't completely worthless.
But what if you don't have or don't want to spend all the required materials for the next upgrade?
I have actually two suggestions, depending on the intention of the system.
The first one is simple. Allow players to "break" or "disenchant" weapons and armor. These items can then no longer be equipped, but can still be traded, and can also be used as special ingredients.
The other suggestion questions the intention. Should players be able to trade bound items after they have been upgraded? Doesn't this circumvent the entire binding system? Are you supposed to level and upgrade all of your armor yourself, trading only for drops? If this is the case, then items created from bound items should also be bound.
I was going to post something along these lines. It is a little silly that a level 10 alchemer cannot be traded, yet a level 1 alchemer MK II, a superior weapon in pretty much every way, can be traded. I understand why the binding system is in place, it promotes continual gameplay, but it disconnects the player a little bit from the "multiplayer" element of the game. I think being able to break down an item into a "material" would be a very good idea.