check out this link and take a look at the checklists.
(Damn, I can't get the links to open - use the search function on this site for "inspection checklist")
Here's a way to get a estimate of value: Price vs. Condition - Vintage Airstream
As mentioned previously, and with all vintage units, you need to know if the frame was inspected and/or repaired, axles replaced - and what capacity, flooring replacement, plumbing - is it pex?, size of grey and/or black water tank... electric winch, new inverter/converter... the wrap probably takes care of leaks
Have they weighed it? Does it come with a hitch system? What are you towing it with?
Take your time and get a real valuation on the "shop".