Sorry to tell you but one strict rule with Sonar is you cannot transfer the licence to anyone once the serial number has been activated. It's a one time deal.
And most people have used that original licence to upgrade so it's would be complicated at best.
There is not much chance of finding an un registered version at this point.
Especially here, everyone here is most likely a registered user with a Cakewalk account that tracks all their serial numbers.
There are many, many pirated versions and your a good person for avoiding the temptation.
But bottom line is 8.5 is now a collectors item and to tell the truth I thought I'd miss it, but I don't at all, I still run it on my old XP laptop and it's instantly harder to use compared to X3 and onward.
You might find it on e bay but you'll be taking a chance that it cannot be registered, As a matter of fact you might find Cake walk no longer even will allow registering older versions.
So in the end it being a legit copy is a mote point.