In short, nothing.
Well, almost nothing. We do allow the listing of Burning Man tickets for sale (for a fixed price, and at no more than face value plus shipping). No trades, barters, rentals, deals, adverts, or other listings. Please see this post on Commerce & Advertising in the Overview of the ePlaya section of the site for more information on that.
We do also allow fundraisers for on-playa projects and theme camps, and you can find out more information about that here.