I've only had one negative experience with Amazon sellers: yet another of these "we sell millions of books!" type places put a hardcover copy of Erwitt's "Snaps" into a plastic shipping bag--no padding whatsoever--and by the time it reached me the book looked as if it had been used to assault a fire hydrant...repeatedly.
I know that Warehouse Deals orders are serviced by Amazon themselves but as for other 3rd party sellers, here are my own personal rules:
1. Ignore sellers that don't describe the book in the description field but choose instead to copy/paste something like "100% satisfaction guaranteed! Ships daily!" You'd be amazed at how many of these sellers don't even bother to describe the condition of the book at all.
2. Huge book stores that sell everything from $.50 paperbacks to $500 first editions and have like 100,000+ feedback. You never know what you'll get or how they'll get it to you (as I found out.) A shipping/handling system designed primarily for efficient dispatch of remaindered or used paperbacks is rarely sufficient for a fine art book (as I also found out.)
3. Look at smaller boutique sellers that DO describe the condition and also have a good feedback history. I've gotten books wrapped in some of the most beautiful paper I've ever seen and shipped with 2 or 3 layers of thick bubblewrap.
4. Check both "new" and "used" links. I find a lot of copies that are described as "new" (and of those I have ordered, all have been "new"), but the seller placed them under the "used" link instead. They are some great deals to be found.