Like it or not, the RV market is cyclical. The same is true of boats and classic cars. If the economy tanks, you likely will not be able to sell any of those for what might seem like a good price right now. If the economy is going gangbusters and you simply can't find a 16' Sport anywhere, you might sell it for quite a bit.
This time of year is a good time to buy in "snow country". The reason is simple: everybody is looking at storing the beast for many months. It's all "cost" and no "benefit" over those months. That tends to put a lot of RV's on the market. Prices go down a bit.
What's a good price for *any* Airstream depends a lot on the fiddly little details. Things like floor rot can have a big impact. There is no quick / simple way to rate the condition of a trailer ( unless there is major damage, in that case .... run away .... fast ...
All that said, if the trailer is the floor plan that works for you, the price sounds ok. I'm not quite sure why you would buy one with the intention of selling it in two years. This is sort of a high price / high risk item for a quick flip.