Well first off buying an unregistered/non-used course is NOT illegal or unethical. Becker can jump up and down about it all they want, but as long as the materials have not been used its a non-issue. I guess the question is how you would go about proving that the seller didn't secretly read the books.
However the law is really on the side of owners here for PHYSICAL items, ie books. You buy a software license, you buy “log in rights”, but physical items you own. The only real way for a company around this is to “lend” you the books for a fee and demand their return at a later time. The books for all review courses are yours to keep, therefore the ownership is transferred.
Legally you can eat your books if you so choose.
The issue, to Becker or another company, is unlikely to be people selling unused materials. The issue is that they want to get paid for use of their materials, instead of having 10 students share the same study pack. If no one has used the material there certainly is no ethicial issue here, nor a legal one. Finding ways around install restrictions (I saw someone selling an entire laptop with all the materials on it on CL) is unethical and illegal.