I presume you have not been lurking long as this subject comes up every few months.

Flyer Talk is very helpful but not known to be kind. I'm probably supposed to make some remark about doing a search and you would find a prior thread, but search hasn't been working well lately and I've been kicked once today myself. Let me bring you up to speed.

The guarantee only applies to published rates, and its a similar deal with most if not all major chains. It does not apply to "third-party" channels, who pre-buy rooms in bulk and resell them. Your complaint does not just apply to Marriott. It changed about five years ago across the board.

Hotels.com, Priceline (at least the bidding channel), and Hotwire are all examples of resellers who buy in bulk and then turn the room for what they can get. No points and no price match. Travelocity and Orbitz used to be the second model, but I'm not at all sure they still are based on my last near-booking experience with the former, when something seemed like I was about to prepay and I bailed for the direct website.

You can usually tell by whether you pay the seller directly at the time of purchase, including taxes, or if they just make you a reservation like a storefront travel agent and you pay the hotel on arrival, just as if you had booked directly with the hotel.

With the economy sucking there seem to be a lot more good third-party deals, as opposed to just dropping listed price. But they are off the record discounts and your reward is to pay less, not to get Marriott to price match,