I had thought about that, but other merchants have their produce marked as theirs and if its marked as someone elses that would mean I could sell his own stuff back to him and he wouldn't notice, right?
Right, but then again there are a lot of glitches with that sort of thing as well, like if you steal the grand soul gem from that wood elf in the Balmora Mages Guild, then you can never use her enchanting services again because all grand soul gems are marked as stolen. So it is possible that you can actually sell someones stuff back to them. I believe there is a glitch that if you drop what you stole, it loses it's ownership or something, so either way you could still do it. But like Adul said, give ownership to someone else, or make another person with the same name, and a slightly different id and give him the ownership, and then place him underground standing on a patch of something or another, if you don't want him really in game, but still want the ownership under the same name. Either way, that is how I have done it, give someone (normally a guard in the room) the ownership, and the items stay on display. A more difficult, and yet somewhat useful, way to do it is to make each display item a unique display item, with no real value or use, just as it sounds, only for display, and then give no one ownership of it, and you won't have any real reason to steal it. Like the Oblivion displays, a sword that does no damage, and has no value, a worthless book with nothing in it, a potion with no potency.
That is the only way I could see to do it, unless..... I wonder if you added the no pickup script to those items, if he could own them, but still not be able to sell them.... hm..... now I want to test that! You know, like the Vivec ash mask has a nopickup script so you can't take it.... unless you want to be able to steal it, then it would be a problem....... Well I tried, lol. Hope I was of some help!