so you are going off dealer prices vs private seller, aka the guy who lives next to you. I agree dealer prices are higher than kbb private sale. The dealer has expenses they pay (salesman, managers, maintenance etc etc ) for the cars on their lot so they charge more than the guy up the street selling his car. Plus the dealer will the tools to detail the car better than your neighbor selling a car plus they are much more convenient. That’s the reason they are able to charge more, I can list my car for dealer prices but I’m going to have a hard time selling it since I don’t offer the perks the dealer has.
Since dealers offer the vast majority of used 2018 SE's on the market, they are a good indicator for a range of what a particular model is selling for in that market. I would always assume a private party sale would go for less money for the reasons you list.