Price vs. Condition - Airstream Values
is a good document that will give you a basis to evaluate the condition and value of a trailer. The items mentioned above are the big ticket items to look at, but let me add one more. Make sure the seller shows you a clear title in his name before you make any deal. Also look at the license plate tags on the trailer. That will give you a good indication when it was last used. A trailer that has been actively used is better then one that has been sitting in one place for many years.
Good luck with your search and let us know how it goes.