Ah... so that's what the "unique" button in Item Editor is for! Silly me, I thought it was actually for making an item unique, like an artifact.
In Item Editor, I duplicated a sword, named it "Shieldbreaker", changed its properties and checked the "unique" and the "merchandise" box, in hope that some day "Shieldbreaker" would show up in a shop and if I bought it, it would be the only one in existence.
Never happened. I've visited 248764276 shops, and no Shieldbreaker.
So what I'm now wondering is: how DO I make an item unique?