Hi, and welcome to the forums. Step 1, figure out how much it is worth. You might do this by taking it to an Airstream dealer and getting an appraisal. Another option would be to look around on the various websites (RV Trader, Ebay, etc.), and see what similar trailers are actually selling for.
Step two is to put it up for sale. Perhaps the most hassle free route would be to get an RV lot that deals in used trailers to consign it for you. That may also be the most expensive (cost of sale) route for you. Craigslist is your cheapest bet, if you are looking to maximize your profits and don't mind dealing with bunch of tire-kickers and low-ball offers. Then there are the commission sites like RV Trader Ebay, and the Airstream Forums Classified ads. The Airstream Forums Classified might be a good bet to expose your trailer to an "educated," enthusiastic, global audience.