1) vBulletin (Jelsoft) is headquartered in the UK. We are based in the United States.
2) The person that tried to transfer their license to you agreed to the terms of our license agreement before they purchased their license.
3) We are a private company and we can dictate to whom we want to license our product and the terms under which our license is or is not transferrable. Our license is non-transferrable. This is for several very good reasons relating to piracy and upholding the value of our product, and is not in any way intended to inconvenience our customers.
4) We do not recognize your license to be valid and suggest you contact the "seller" to obtain a refund.
While buyer beware certainly plays a part here (as stated, the terms are in the license agreement) I do feel there should be a transfer option of some sort even if it includes an "administration fee" or reduced rate for existing installations. When the day comes that I sell off my site, all content and software will be transferred to the new owner but whoops, you will have to go buy photopost again to run the existing gallery section whether or not you want to download updates and/or receive support or not. Perhaps you could revisit this issue as I'm sure I'm not alone in thinking so.
When someone sells their entire website to another party, we consider allowing the transfer of the PhotoPost license on a case by case basis. But we don't allow people to just sell off their PhotoPost license in and of itself.