I got frustrated once that my local supermarkets carried all but one particular flavor of a certain (food) product. So I looked on Amazon, and somebody was selling a moderately large case at an inflated price, and I bought one, because there didn't seem to be another option. Within a few weeks, I see the same thing on the shelf at a local store, substantially undercutting what I paid. Then, however, they never restocked it.

It wasn't like I was planning on starting a business of reselling it, but it was startling.