Now, I do like that idea Malcolm. I think what one could do is send them an e-certificate, with a # on it. There would be a bit of bookeeping involved to keep track of the thing, but I don't think it would need to be difficult.
I like that idea because the person is encouragd to come back to you, the original seller, instead of some other seller.
On a very basic spreadsheet you couldjust record a number associated with a particular id and once it's redeemed, you would just notate it on the spreadsheet. It would be pretty easy I'm sure.