Hello,
No, you cannot buy a license to own from anyone but us.
It's mentioned here quite clearly:
http://www.directadmin.com/internal.html
The only way a 3rd party/Reseller can give you a license is if they provide you a server and include DirectAdmin on it. The license belongs to the Reseller and not you, you're only "renting" it from them for use on their server.
If you buy a license from ****************.com, and they don't give you a server with it, you do so at your own risk. You risk losing your license, since it's not yours, as well as your money.
Keep in mind, we do have the ability to nuke servers that are not running legal copies of DirectAdmin. If you buy from a 3rd party Reseller, and didn't get a server with it, you would fall into that category, and I'm pretty sure your clients wouldn't be too happy if that happened.
I've also made it a "bad" word, since we don't want to increase their search rankings.
John