Yes, that sounds right. It is the product key that is sure proof of ownership.
Let's review what happened here. Was it something like the following?
Number 1. A user somehow acquired a used computer made by one of the major manufacturers. It has a product key on the back panel that indicates it had a Vista installation .
Number 2. There was no installation DVD in the transaction of the computer to the new owner.
Number 3. the new owner buys a copy of the DVD used to install the OS on this brand of computer. He uses the given product key and everything is okay.
WAIT! What did he say?
Purchased System Builder OEM disc.
Nor sure. You may have to call Microsoft.Edit: Here is a link.
http://oem.microsoft.com/Search for 'System Builder'
Results vary by region.