I believe you have summed it up nicely. That's all I was going to say.
Unless you disassemble the laptop in question or know for a FACT (via manufacturer / OEM specification) what it uses I would just forget about it and move on. Ebay is about 60-75% reliable in regard to telling if a particular part will work in the desired machine.
So the safest thing to do is wait until a machine with the graphics you want becomes available and buy that. Or you go the barebones route (Compal, Clevo, ect)