[jedberg]

Then you’re back to the original problem of having to perfectly remember what you bought and for what price, otherwise you’re committing fraud.

[dheera]

That's easy, because the prices might still be physically listed somewhere if it's a store, or you might still have the Craigslist email thread, or whatever.

If you simply lost or don't have a receipt and it's done in good faith I don't think it should be considered fraud.