Now, the site I bought it from has since gone offline and the person I bought it from deleted his email account and I lost the paper receipt, but there's my certificate of authenticity.
You have a url that purports to be a certificate of authenticity. What happens to that certificate of authenticity when the site goes down, or gets sold to someone who decides to replace all the images there with poop emojis?
Same thing that happens to all my art when my house burns down
I don't like NFTs personally (wasteful, scam-ridden, investor bro culture), but if you compare them to how art has worked for the past century (or more) it's not so different. If you own a Banksy the certificate of authenticity is far more valuable than the actual artwork, which is easily reproduced. It's all kind of silly.
If they promise you to own that piece of media, and then later make it unavailable for you, then yeah, that's a scam. DRM, or things like DVD region locking are also quite scammy if you ask me.