This situation is the same as seeing someone drop their wallet and start to walk away. I recently sold some stuff on ebay and was totally paranoid about what would happen if I made a shipping mistake - I really needed that money.
I would let the seller know, otherwise I'd be f*cking a potentially nice person out of a couple hundred bucks. If it was a scumbag reseller, I'd still let them know (although I'd probably wrestle with my conscience for a day or two). Keeping the item would require judging a stranger and, as much as I like a good flame war, taking someone's money crosses a line for me. Ebay history can tell a lot, but it doesn't tell you if the seller's spouse has cancer, or their house is about to be foreclosed, or if they just got laid off, etc.