That is an interesting topic! I've bought a bunch of used cars, though most have been sale-by-owner from private citizens. A small handful have been from used car lots, but none of my used cars have come from new car dealers. I've looked at some used cars at new car dealers, but never bit.
Usually, when I was looking, I was looking towards the back of the lot. These were cars they weren't going to put a lot of money into to sell - the cheapo trade-ins that they might just end up selling to the wholesaler. When I did look at the nicer cars, I have usually experienced very clean cars. If they were asking over $4000, it was usually cleaned up very well.
There is a local car dealer who buys car detailing supplies from our parts store. They only want certain products, because they believe in them and get the best results. Their used cars always look great. I've checked out a few when I'm dropping off parts, and since they are one of three dealers right near my home, I see a LOT of cars from them, new and used. They are always very clean and well-prepped.
As a buyer, especially if I were looking to buy a used car from a new-car dealer, bad prep would be a red flag to me. If you are trusted with perhaps millions of dollars in inventory, plus used cars, there should be no excuse but to prepare your inventory. It should wow the people who come to look. Especially if it has a certain price tag. Even if it's a "back lot beauty," it should be clean and presentable, even if you don't waste Armor-all on the cracked dash.