As I said, it's not something I would do unless I knew more about the dealer. The prices in question are just way, way above what I would pay. Or what I've recently paid for similar (though not identical) issues. And way over (3x or more) guide for not-at-all hot books though, admittedly, also not easy books to find.
Similar situations crop up from time to time with books listed on the boards. I figure there's no point in making an offer unless you're ultimately willing to come in at least 75% of the original listing price. But then again, someone may just be listing at a crazy price hoping someone jumps.