If one is has, say, a broken Tinkertool.
And there are two in the bag, one of which is NOT broken.
And one is attempting to sell the broken one back to a vendor.
How does the vendor know which one to buy?
Thus far, I've been putting dupes on the ground if I want to make sure I don't sell them, but I'd hate for someone *coughcoughdondrapercoughcough* to think they were up for grabs before I could retrieve them.