The only way to know if you're getting a good price is to do LOTS of research in your area. NADAguides.com isn't gospel, but it's a good starting place for pricing used RV's. Do not add in any options when valuing used equipment.
I also much prefer buying and selling, well, almost
everything (including RV's) gently used from private sellers. I had overall good experiences and got great prices on both of my RV's from private owners (found 1st motorhome on Ebay, 2nd trailer on Craigslist) but again I had done tons of research on what I wanted, and what was out there. It's all about timing, and knowing when to jump on a good deal when you see one.
Negotiations-wise, with RV #1. This was in 2007 and the unit was a 1994, 13 years old at the time. It was valued at $17k retail, but seller was motivated and had starting price on Ebay for $14k. I watched the first auction end with no bids, then get relisted. At that point I contacted the seller and set up a phone call. We talked about everything you could possibly discuss regarding the RV, probably 2+ hours of conversations over 2 phone calls. He knew I was a serious buyer. During that call, he mentioned that $12k was the lowest he could accept and I agreed. My wife and I drove across 4 states (IL to NY) which is whole 'nother story that is detailed elsewhere on this forum.
RV #2. It was in 2015 and unit was a 2008, 7 years old. Valued at $14k, again a motivated seller and I had seen it advertised on Craiglist for $11k, then dropped to $10k. Followed the same process, called/emailed the seller numerous times to ask all kinds of questions and talk about the rig and the owner's reasons for selling. (The seller told me she appreciated all my questions, because she knew that I was serious.) With no deal made yet, I drove a couple hours to see the trailer and we talked for a couple more hours while looking at everything. It needed tires and an awning, so I mentally discounted for that. Basically again let the seller tell me what she would take, and we settled on $8500 and I towed it home... with no trailer brakes and a borrowed 1/2 ton truck in suburban Chicago traffic! (I know a lot more about smart towing now.) The tires later cost me about $450 and awning was around $600 and I installed myself.
"Watching and waiting" are the keys to a good deal on anything. Do the research and know when a the perfect item/price comes along. Most used RV's at dealerships were way overpriced in my area so I never went that route. I have bought several used cars at small dealers though, usually older stock that they want to move off the lot but was perfect for me.