Yes, used versions are usually okay as long as the original owner is deleting all the content and transferring the license to you. However, if the original owner has used their copy of PS7 to upgrade to PSCS, then you wouldn't be able to use it in the same way to get an upgrade. If you have any queries about the legality of used software, I advise to contact the seller. I have found that if they are selling illegal copies (which they'll sometimes call "backup" copies or whatever) a simple query of "Is this a fully legal copy, can it be registered with the publisher?" will get zero response. That's what I've always found with e-bay, anyway :-

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire