Look at the second link I put up, the FAQ for the ad agreement...
In Question 17, they make it clear that they don't care what the stock is worth (they'll even take nonsalable inventory!), they will give "full price" - or even higher - (in media credit equivalent only) for assets that may only be worth 10 cents on the dollar, or less (or nothing at all!)
They don't care, and Question 28 holds the answer... in order to USE the "media credits", you have to pay an administration fee of around 5% - in CASH (or free trading stock, which means something they can sell for cash).
My guess is, they mark up their wholesale ad costs by 25 times or more... e.g., if you want to actually use the "media credits" that will cost them $100,000 (like an 'advertorial', as they call it, in newspapers across the country touting your company), they could mark it up to 2.5 million in "media credits" out of your account - accompanied by, of course, $125,000 in cold hard cash!
So, they make a profit on the cash, use up your credits and they keep the stock or inventory as well (which they don't really care about, because they already made a cash profit - but who knows, the stock or inventory could be worth something someday too)
Actually, it reminds me of a barter system, writ large.
(Keep in mind, this is only a guess... but it would make sense, and would seem to be legal)