Pete, you have some good information given to you already. I did not know of, nor was I privy to NADA guide line information when my wife and I purchased our 1980 21 foot FunChaser.
Just prior to posting this, I entered to the best of my knowledge, information I could, about what we brought home, and was satisfied with the suggested retail price we paid. Our trailer needs a lot of TLC, but it has a reasonably good foundation to work on. That much I considered when we looked it over.
To feel sorry for the previous owner... Yes, they could well be proud of its appearance when they offer it up to someone else; however if you have doubts about the asking price, you might think about reselling it after you put your time and energy into making it better.
You too, will more than likely feel like you are giving up one of your family members. Let logic rule.
As SAM said, It is a buyers market, and there other trailers out there.
Best of LUCK to you... Dabbler