Can't register my CS5 license into my adobe account

>sells bulk licenses as an original and unused license

That is not legal and Adobe will not recognize the purchase

you can no longer register old products like cs5. 

and you probably purchased an illegal license so if it's activated on your computer you should consider yourself more than lucky.  if you purchased it recently, it is an illegal license.

if you want to verify the above, you can contact adobe support. there are 2 ways to contact adobe; chat and twitter:

chat:
use a browser that allows popups and cookies
and click here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human.

twitter:
tweet @AdobeCare

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true