As long as a license isn't an internal license it can be resold. When buying a license you should always make sure the license isn't an internal license. You can tell by logging into the admin control panel and checking the license details; internal license say so. Or you can check with DirectAdmin once you've gotten the UID and the LID.
You'd still have to talk to DirectAdmin staff once you buy a license; it has to be transferred to your account, and to your IP# and OS distribution type.