PTF. Looking to buy a car and pay cash ($20k all in) but can't find good private party deals and seems that online or in person dealerships might be the way to go. Curious if anyone has details on the best way to go - online vs. dealers (wholesale, used or New dealers)?
It really depends on what you're looking for, how much negotiation you're willing to put up with, and whether you're willing to finance (even if it's only for a month).
To the first point, dealers know that the market is especially hot in a few categories: trucks, fuel efficient cars, electric cars, and off-roaders. If you're shopping in any of these categories, you're really going to need to do research ahead of time (KBB, NADA, cross-shopping other regions online), because whether you're buying in person or online, prices tend to be higher in these categories.
To the second, the absolute best benefit for buying online (or from Carmax) comes from the fact that the price you see is more or less the price you'll pay. The downside to that is unless you're looking for a dime-a-dozen small to midsize crossover, the prices at Carvana/Carmax are going to be a bit higher than what you would expect based on private party listings or blue book values. If you have the time and willpower, there is usually a deal to be had if you look carefully enough and are willing to not only just negotiate but also to walk away from negotiations with a dealer. You won't be getting a screaming deal but it's definitely possible to get lower than Carmax, just with added hassle. When I found our van, it was sitting on the lot at a Chevy dealership that heavily favored sales of trucks and fullsize SUV's. Nobody browsing the lot in that demographic would have had any interest in a Honda minivan and so it sat undriven for a couple months. Had it been at a typical lot or a Honda lot it would have been sold within a week of listing.
To the third point, if you do happen to find yourself working with a dealership, as was mentioned upthread you're going to have a much better shot at a discount if you finance and especially if you finance with a bank that the dealership chooses. You can absolutely make sure that the loan terms enable you to pay off the loan within a month, but because the dealerships get incentives from the bank to sell financing, they're much more motivated to wheel and deal if you're opening that option for them.
FWIW, my parents have used Carvana twice and have mostly been very satisfied with the service, but they also paid ~$1500 over what they might have been able to get by shopping in person.