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.

I can't vouch for the ability to combine the Ebay offer you cite with cash back, but typically, two forms of cash back/rewards aren't stackable. However, if they are in this case, I'd suggest using the BigCrumbs cash back of 4% over eBates' 3%. (My referral link to BigCrumbs is in my signature, thanks!) Also, there are currently offers for a 10% bonus deposit on gift cards (promo code MMF at checkout) and an offer for 10 MR points per dollar spent (up to 25K points, no promo code needed, and not stackable with the 10% bonus). Both of these should be stackable with *one* cash back/rewards offer, such as BigCrumbs. If you have a Chase Visa Marriott Signature Premier card, I suggest using that for yet another 5 MR points per dollar. Last, when you actually book your room, use BigCrumbs for another 4%, and of course, check on AAA rates versus other room rate codes. How's that for simple?!

if you want a 9% discount on a marriott gift card, contact me. I will sell you as many $200 gift cards as you want for $182.00. $100.00 gift cards for $91.00.

No thanks. Stacking the aforementioned promos would get 14% off + 5 MR points (or 4% off + 15 MR points) - much better deals.

eBay bucks is built inside the eBay system, anyone who are member of the program will automatically earn cashback without the need of special link, tracking cookie, etc. Therefore you can also get cashback from 3rd party. I've been in the propram since it started so I know it works.

So you're saying you can click through a BigCrumbs (or other cash back site) link to get your 4%, arrive at the relevant site for this promo, and still get both offers on top of the MMF or 10 point promo? If so, great! That would make the total take 4% from eBay, 4% from BigCrumbs, 10% off MMF promo code, then another 4% for booking your room. Add the 5 MR points per dollar from using a Chase MR Visa Sig Premier card, and you have 22% off plus 5 points per dollar, and that's before even considering points earned for your stay (regular + elite bonuses + megabonuses + etc). Wow!

Is this for real?!

The eBay and the MMF and other gift card promos are no longer active, AFAIK, but otherwise, yes.