Hi Greyhound –
From my limited experience, if Jay signs more than one piece, just ask for that many certificates. He'll give them to you. As far as re-selling his items with only the signature, check out “Strongwater” on eBay…no one sells their items with a COA, which tells me they weren't signed in front of them. I'd check the signature, since you already have the real thing, against what they send. But his signature on items does add to the value, certificate or no certificate.