As you may know, in addition to the items Amazon itself sells, some of the items that are found at amazon.com are in fact sold by other vendors: Amazon collects the money, but the order is fulfilled by another vendor.
Today I discovered an instance where Amazon's price for an item that would be supplied by one of these other vendors was noticeably higher than the price I had just seen on that other vendor's own Web site. I don't know how common this is, but it's something to keep in mind.
Perce