I work in auto parts, and while most transactions are mundane we do get some... oddballs here and there. One night, a gentleman came in saying he needs a battery, and he brought an old one in. I think this is great; I'm gonna net a ~$150 sale and he's already got his core in hand! I see it's one of our batteries, a two year unit, and presume either he wants to warranty it or it's actually going to be his core.
I inquire about it and he said it's a good battery and he's had it about a year and a half, but he doesn't need that one anymore. The unit he brought is a top post unit, and he now needs a side post unit. That's straightforward enough, so I show him our side post units and their prices/warranties. He says "No, I want to trade this battery for one of those." This request is, of course, absurd, and I try to be polite in telling him, "No, you're a moron, buy something or get out," but he's not having it.
He insists I can take that battery back after 18 months of use and resell it as a new one, and all I've gotta do is give him a new side post unit for it. I ended up having to tell him I can't sell that battery and the best I can give him is either a $5 recycle or $18 core credit on it. He then retorts that if he went to any other [our store] that they would accept that deal, as if I'm somehow going to buy that.
Thankfully, he wasn't rude or anything, but he did get rather offended when I told him I can't trade an old battery for a new one. He ended up leaving without buying anything. I still wonder what possessed him to think that idea wasn't absurd.