Adobe's web site describes how to transfer a license: http://www.adobe.com/support/salesdocs/1086a.htm
The person selling Photoshop to you has to cooperate in this process by signing the transfer form. If (s)he has used the copy of Photoshop that they're selling to you to upgrade to a later version, they're not allowed to sell it to you. And, of course, they have to uninstall the product that they're selling to you.
I've gone through this process myself, when I sold a copy of After Effects 5.5 and Premiere 6.5 to someone else. I took advantage of a special Adobe offer to buy a full (i.e. non-upgrade) version of their new Video Suite, and so wasn't upgrading these older versions of the products. That's one example of a legitimate sale.
Watch out for academic versions. You can't legally use them for commercial purposes.
Adobe makes the process very straightforward, using the forms on their web site.
Hope that helps!