Don't groan - I'm sure this has been done to death! I can't really afford AeCC so I'm looking for an old copy of CS6. There is a version on, yes, ebay, where you download from helpxadobe, so presumably it's a genuine version, but the serial number is provided by the seller. Is this legal and will the version work forever? I know I can't upgrade it.
Before purchasing another license from another party:
Please read this doc: Transfer an Adobe product license