Let's suppose someone is selling their complete package (including disks and serial numbers).
If the package is used, you must get a transfer of license form, completed by the seller. Otherwise they have not transferred their license to you; they are still allowed to run it, and you are not allowed. Adobe cannot help without a transfer of license.
If the package is unused, be very wary indeed. Ebay and the web in general are a hotbed for fake product. If someone seems to sell more than one copy of an old product, ask yourself where they get something that Adobe stopped making years ago.
Also, CS6 is unlikely to work with future systems, especially on Mac. Really, consider subscriptions, or consider a different product.
