I start to wonder - if many of us have had this experience, then why aren't we getting into the business and selling?

. Seems that there is a market on this thread.
Some car dealers in our area simply advertise their lowest guaranteed price. I know folks that go to them for this very reason and have had a very good experience. Here is an example.
http://www.wilsonvilletoyota.com
The challenge is that typically the price is one that puts everyone on an even playing field. Many like to brag to friends about how they beat the system. It's a trophy.. Rather than some getting messed over and others getting a great deal, all get largely the same deal. Yet the margin may be higher than what you want to pay and folks game the system by walking into other "hotbox" dealers with this as their starting price for a haggle.
Thus, dealers like this eventually give up the "one price" and go back to the old system.
I suggest that some of the challenges we face at dealers we bring upon ourselves, myself included.
On the price item that others shared, the past 3 vehicles I purchased I largely settled on price before walking in the door. It removes the emotion for me and I find that modern dealers have an internet salesrep that understands this. That works well for new as unless there is damage, one can predict exactly what condition it will be in. Used is a different story.
I can't afford new RVs and the depreciation is incredible. Condition is very important as many of you bring up. Private parties can be dishonest too. I've been tricked even after due diligence. For everything I sell, I share every flaw about my rig and price it accordingly. Buyers appreciate that and I sell quickly as a result.
Most cars I buy new. That works just fine, provided that you keep them at least 10 years. That's what I do and the next person gets an awesome used rig.