COAs usually can't be legally sold without the machine (There may be a few, rare cases), but for the most part, they're not desirable.

Windows 10 can be installed over 7 or 8.1, so anything before then (Such as an XP COA) would be completely obsolete anyways (Plus, how many XP PRO COAs are out there, anyways?).

Now, if you're talking about COAs for stuff other than Microsoft Windows operating systems, that's another story entirely....