Can you open a box before paying for it? It might be full of junk, or bricks, or some other items of similar weight.
This whole thing reminds me of the old scam where a guy drives up in a van full of boxes, all supposedly containing high-end car speakers, or TVs, or stuff like that. "Supposedly" the whole van full was some sort of leftover stock ordered by mistake and the guy just wants to get rid of them for a too-good-to-be-true price. Cash only, of course, and "pay quickly before the boss catches me!" He might even have one box already opened so you can see the speakers or TV or whatever. Then the guy drives off with your money and you're left with a cardboard box containing a couple bricks and a bunch of bubble wrap.