I think the Airstream Classifieds here on Air Forums is a very good way to market your Airstream. A good description, good photos, and prompt feedback to prospects are necessary. Be willing to allow a 3 party inspector take a look to build the buyer's confidence. I browse the classifieds frequently.
I happened to buy mine on e-bay but I did not like their process nearly as well. I'm not much of an auction guy: hey bidder, bidder, bidder.