i purchased a used version of 8.5 sonar can i register it under my account

As long as the product has not been registered before, it should not be a problem. Here is the registration FAQ http://www.cakewalk.com/register/faq.aspx
 
If the product has already been registered, Cakewalk does not permit license transfers.

IT HAS BEEN REGISTERED BEFORE
THANK YOU

you need to speak to cakewalks customer service and see what they can do for you *IF* anything, they aren't any obligation. the person who sold you it is though.
 
if you bought it off an auction site for example, some offer payment protection. I have had full refunds on a few things that were badly misdescribed.. they forced the seller to refund on them.