You can't re-register a Manhattan GMAT book. However, if I person bought the whole set, it's unlikely that they registered each book (I've only registered one of my seven), so if they can tell you which ones they haven't registered, then you can register one of those and have access to the tests.

Otherwise, they'll have to pass on their online account, which they may not be willing to do.

I'll be done with my MGMAT books and OGs in about three weeks if you can wait.

If they pass me their account, can I re-take all tests that they've already taken?

Also, if you register one book I assume you only have access to the tests for that specific book.

Thanks!

Yes, you can reset the CATs.

Each book gives you access to the same CATs. More exactly, each book gives you access to six tests that draw from the same question bank. So if you register two different books, yes, you can take 12 tests, but they're going to have a lot of repeats. It's equivalent to taking all six tests, resetting them, and retaking them.

In addition to the CAT access, registering a book gives you access to additional questions for that particular subject. So you will lose out on that if the seller won't transfer the account. I haven't used any of the additional questions, so I'm not sure how useful they are.