Sorry, what about that would be dumb? Attempted reuse is exactly what would be prevented by scanning the barcode and seeing it had already been used.
This is exactly why gift cards have scratch-off material covering their secret keys, otherwise folks would just take a picture of the next card in the stack, wait for it to be funded, and then use it before the recipient does.
edit: And would you look at that, you can buy stamp sheets on Amazon.co.uk, and users have helpfully submitted a bunch of photos with barcodes visible.