I think reset to factory default unregisters the Kindle. It should, after all, since there is no registration at the factory. Personally, I would delete all the books, sync, then reset to factory defaults.
Except that if I order a Kindle through my account it arrives from the factory already registered but with no books.
That said, I remember when I reset my DX to factory to give it to my aunt, and then went to de-register it, I found it already was. I remember because I was surprised by that. So I am still not sure.

But I do think sebat is correct in that if you de-register and THEN reset to factory, the licenses might not be freed.
Note that, to be sure the licenses are freed up, you should delete them using the Kindle. And, as Heather suggests, then connect wirelessly and do a sync. If you just delete the files while the Kindle is in 'drive' mode connected to your PC, I'm not sure the licenses will actually get freed up. So you either have to do them one by one, or do ALL at once with the 'reset to factory' option.
In other related information: if there's no device limit specified on the book's page, it's 6. If it's something else it will specify how many licenses or say 'unlimited'.