These are probably stupid questions, but anyway, here we go: 

What are "cozies" [in regard to a type of book], and

I have seen many references to listing unpostables in the Book Bazaar for a credit, but I've never actually seen such a thread, at least as far as I know.  Do you just write something like, "I have an unpostable copy of Book X available for a credit, please PM me"?

edited to correct punctuation

My interpertation of cozies is a light mystery such as Carolyn Hart's Death on Demand series, Cleo Coyle, Diane Mott Davidson etc. 

I think the only time you can get a credit on BB for a non-postable is if it is WL'd. I think you can also put it in the genre forum as well.

There was an excellent explanation of cozies in this thread:  http://www.paperbackswap.com/forum/topic.php?t=65519

You can offer any unpostable for credit in the BB.  It's just very unlikely that anyone will take you up on it unless it is WLed.  You can also post in the genre forums, in the appropriate threads. 

There is an ongoing Unpostables thread in the BB but it can be hard to find sometimes and it's really long now. So you can jsut post what you have ut like caviglia said it's unlikely anyone will paya credit for a book that's posted to PBS. BUt they might for a WL book especially if the line is long or it's hard to find. I usually just offer htem as freebies wiht an order from my shelf. I also joined the Damaged Books Virtual Box which is in the games forum.  It's a group of us that exchagne well unpostable books.