From the Q&A (https://www.marriott.com/hotel-rates/faqs.mi):

10. Can I submit a claim based upon a lower rate found on any Web site?
Most Internet Web sites are included in Marriott's Look No FurtherŪ Best Rate Guarantee (for example, Hotels.com, Expedia.com, Travelocity.com). However, the Web site must be one in which you can complete the transaction on the site. Opaque providers of hotel rooms, such as Priceline.com and Hotwire.com, that do not reveal the hotel brand name until after you have pre-paid your hotel room do not qualify for the guarantee.