Once an RV reaches about 15 years of age, condition becomes all-important and age ceases to be a factor. There is also a wide price spread among brands, e.g. Cadillac vs Yugo. Premier brands can easily run double the run-of-the-mill models, but as a newbie you probably don't readily recognize the names or the differences in construction and amenities.
And then, as Captsteve says, private sellers often just overprice them, at least initially. They are often in denial about the tremendous loss in monetary value they have suffered.