but he does have it listed as no returns.
He probably knew why. Let's hope you get the disc and the key. Then you would be in business. In the meantime I would try to read what is on the disk - maybe stick it into an external enclosure or into a desktop. If you discover any manufacturer name like HP, Dell, Gateway or whatever on it, then it is an OEM system with which you can really do nothing - and the seller will not have a disc or key.
it is possible that the seller is a nincompoop and did not even know that the disc is of no use to someone else.