NADA is one source, but it does not reflect local variables. Some models & types are more popular than others in a given region and the local prices will reflect that.
Private sales generally go for low NADA retail or less, but f you have a popular model that can vary too.
As Ned suggests, check craigslist, ebay, and the numerous RV classified ad sites to get an idea what people are asking and paying. Don't rely on just one or two samples, though, because many people overprice initially and don't get "real" until months of no-sale go by.