they're tradeable, so if you wanted to sell them, you'd just give that person however many he/she wants in a trade and you'd get zeny from that trade in return.
however, since the page is down right now, what people are doing is they'll do the trade and put the zeny for it, but the other person will put in nothing, you'll give them your account name and number (you'll find this on the ad credits page of the website), and wait for the ads to show up in your page (they would have to use the 'transfer' tool to do this), then you click "deal" for the in game trade and the other person gets the zeny.
I hope that makes sense, I'm half asleep right now .__.