ebay has promotion: earn Double Bucks(4%) from Giftcardmall, which sells Marriott Gift Card.
If you combine this with ebates.com, that will give you another 3%. Using a reward credit/debit card will add another 1-2%. you will get at least 8% off.

For those who use BofA, you can use Add It Up program instead of ebates.com. However, you will have to use BofA debit/credit card.