The seller of the program MUST transfer the "License" (EULA) with the program as well or you will have "physical" possession of Photoshop by unable to register it.

As stated too, if it's an upgrade (though the CD does contain a full program) it will look for a previous version installed on your drive before continuing with the installation.

not sure about the trust factor when buying older commercial software on ebay, but DEFINITELY steer clear of any "OEM" packed offers!!