We have used them also. I think it was last year around this time Marriott did a promotion. When you ordered a Marriott gift card, you got $10 in Bonus Bucks. Of course, their intention was for you to give the cards as gifts. But I just ordered a bunch of gift cards for myself and got the Bonus Bucks for free.
As for where they are good... apparently there are different kinds of Bonus Bucks. The ones I had clearly stated, though in tiny print on the back, that they were good at all Marriotts, Fairfields, Courtyards, SpringHills, etc. When I presented it to the front desk at a SpringHill, her first reaction was that it wasn't valid at that hotel. When I pointed out the fine print, she showed it to the manager and they honored it just fine.
So if you are buying them on ebay, which seems like a great deal to me, just be sure that it is the kind that is good at all Marriott properties.