Exactly!

The books are all a guide, not a Bible.
And as a dealer, I have yet to see any softening of prices since Covid hit. There is less selection of used cars, and the nice ones do all the money.
I tell people all the time, if the price I pay to advertise a unit isn’t good/ competitive , you wouldn’t be here. Cars would sit around and rot, nobody comes, nobody calls.
I see stuff all the time that approaches and or exceeds retail, at dealer auctions. Then you have auction fees and transportation and servicing on top of that..
We have had a hard time keeping as much inventory on hand as we have in times past. It’s just hard to find stuff that leaves enough meat on the bone to where everybody wins..

Now on average is the selling price of a used vehicle by a dealer (or private individual) slightly higher than the "blue" book ("guide") values with the current market or the same or even less?

The true actual value of any vehicle is what somebody it willing to pay for it. Vehicles in high demand in excellent condition usually bring a higher value and higher profit margin than those that are not. That's why it's tough to discuss this in generalities. Those that have access to the Manheim auction reports have seen that over the last several months, used car prices have decreased between 10 and 15%. The hard and fast rules of value OP is asking for just doesn't exist.

Ah "OP" that would be me

2 weeks ago a lady that works with my wife was offered.
4K trade in at dealer she got her new car at. Carmax gave her 6K, and her husband said private seller buyer would be 8K if cars,was 100 percent and it had some stuff that needed addressed s.
I guess Carmax was,spot on.bet they put it at 8-8500 on the lot and fix a few small things

I also wondered how pricing is being done these days as some of what I see has been pretty crazy since 2019. Seems like if you wanted to take out a loan at a bank by yourself you would have to convince the bank of the street value in real time vs a guide that may not reflect what is going on out there.

Like how does the local Dodge dealer buy back a used low mile 2019 Challenger R/T they sold a guy I know for even more than he paid for it about a year later. Then the dealer resells the same car they bought back again for even more money as they had a buyer lined up for a clean used one. Then this guy who sold the dealer his car back takes that money and buys a clean used Challenger T/A at a different dealership for almost nothing more out of his pocket. Two cars traded hands and the 1st dealer made out, and the guy got a better performance model Challenger for almost a wash. 1st dealer swung for the fences on the R/T and found the right buyer and the other dealer may have gave the T/A away too cheap?

If NOX standards disappeared overnight so would EGR systems, A/F ratio at cruise would probably go lean a couple points and compression ratio would go up at least a point on almost every engine and some would go up 2 points or more and would all probably result in at least 10% MPG jump and probably more likely 15%. CO, CO2 and HCs all tend to go down when efficiency goes up but cats would be ditched and so the actual tailpipe emissions would go up even with less of them coming out of the engine itself. I don't think modern cats cause enough flow restriction to make much difference in performance or MPG but they don't like running lean so they would have to go in an all out MPG search.