Start by researching similar units on the web. RV Trader and RVT have thousands of units listed and many of the dealers have their policies listed as well.
Dealers are much like a used car lot, yes some are shady, Ask when you start negotiating for all their fees and add ons before negotiating any further. Some will offer a great deal, and then tack on $399 for documentation, $1500 or more for prep, and then want another couple hundred for your walk through session to familarize you with the unit. If they have all these fees keep those cost in mind as you determine price.
On a used unit remember that on a private sale, those cost are gone.
If you do decide to go with a dealer, find a location which is locally owned and has been there long term, the need in the future for problem resolution and ability to actually get to the owner is valuable.
Enjoy the journey.