Of course you can buy a used one, if you find one (not to mention an unused one)! Why would anyone say otherwise?? You just can't register it, and the purchase is against the EULA. IIRC you can also install it and use it, if it comes with the codes.
IMO it's just silly to make a noise about "innocently misusing" a 20 year old software. Actually, that's  taking care of a noble heritage :o)
I think you do need an ooold OS to run Pro Audio, at least the earlier ones, though.

Anderton

John
If its unregistered it wasn't previously owned. 

It could have been previously owned, but not registered...

So, As I stated. Yes! If it's an unregistered version.
 
Also... 
http://www.computerworld.com/article/2505356/it-management/eu-court-rules-resale-of-used-software-licenses-is-legal----even-online.html 
and...
https://www.techdirt.com/articles/20120703/11345519566/eu-court-says-yes-you-can-resell-your-software-even-if-software-company-says-you-cant.shtml
 
Just saying, although I can't think of a reason for wanting ProAudio other than nostalgia 

IANAL but I do not believe the EU court rulings apply to most here. Most of us have a single license to use a Cakewalk DAW which may have been upgraded over time. If an older copy is sold or given away the license would need to be included so recipient could use the software. Failing to surrender the license would result in multiple users of the same license which I don't believe is OK anywhere.

BENT
 
Just saying, although I can't think of a reason for wanting ProAudio other than nostalgia 

It's the vintage sound you just can't get from new code. 

Correct me if I'm wrong, but in the Pro Audio days all you needed to use the software was the serial number that came with the package. So, all this talk about the registration etc. doesn't have anything to do with the usability of the illegally purchased software. Used or unused, you just install it, add the serial n:o and start working. I registered my first Pro Audio in the early 90's with a postcard several weeks after I had started using it. No internet.
I still think that if someone bothers to keep a 20 year old software active it just serves this whole hobby/profession of ours.
Otherwise, I'm rather strictly against pirates. And don't ask me what's the age limit for the software to my not accepting illegal purchases :o/

Kalle Rantaaho
Of course you can buy a used one, if you find one (not to mention an unused one)! Why would anyone say otherwise?? You just can't register it, and the purchase is against the EULA. IIRC you can also install it and use it, if it comes with the codes.
IMO it's just silly to make a noise about "innocently misusing" a 20 year old software. Actually, that's  taking care of a noble heritage :o)
I think you do need an ooold OS to run Pro Audio, at least the earlier ones, though.

Yes true, but I also want to know if it will work and if someone will sell it as well. So the question if it is legal is unavoidable on the forum of the manufacturer 

I remember several years ago discussing this issue with someone from Cakewalk's customer support service. I was surprised how actively Cakewalk pursue individuals who sell older copies of Sonar. It helps to ensure the provenance of an item and that the sale won't contravene Cakewalk's licence agreement. They will and have taken legal action to protect their rights.

Brando

BENT
 
Just saying, although I can't think of a reason for wanting ProAudio other than nostalgia 

It's the vintage sound you just can't get from new code. 

Silly me, didn't even think of that.
In Features & Ideas, I'm going to suggest Cake develops a ProChannel Module to emulate the 1990's hexadecimal warmth that you just don't hear these days.

That's what my recordings are missing. And I though it was me!

Kalle is correct. Software, CD key and S/N were it... There was zero requirement to even register the software.