I have ordered many ( books , photo gear etc..) things from Amazon that were sent directly from Marketplace vendors. You do have some protection as Amazon will stand behind things ordered from their site. I do not know the full extent of those protections, as I have never fully read the rules. Most of the camera stuff I have received that was not sold by Amazon directly came from regular established photo shops such as Adorama etc... never a problem yet.
edit: I always look at teh star ratings that customers use to rate a vendor & participate in rating when I can