You should probably make a photocopy of the receipt (with your personal information blacked out), and you should state this in your auction. I don't think it's safe to actually give the buyer the real receipt (and who knows what people can do with them?).
even receipt can be faked on Ebay, i would actually prefer to keep the original should the buyer claim it's not real. Most auction only supply a copy of receipt w/personal info black out, so it's quite acceptable practice.