Tino - if you click on the offer at Volumebuy's home page it lists all the certificate rules. Essentially they are as good as cash (with some limited restrictions), but you essentially make your reservation as you always do, and simply pay with the certificates on check-out. Thus they are not good on the "advance purchase" rates Marriott offers from time to time but otherwise seem pretty flexible. The pool closes in 2 days - and has now dropped to 47 more items need for the lowest price.