As someone that just bought two cars:

Autotempest.com

It's an aggregator for used car sites such as Cars.com, autotrader.com, ebay.com and a couple of others. It has a pretty decent set of filters that works on all the aggregated sites.

Fb marketplace.

Craigslist

Somewhat dependent on budget, but don’t forget about FB marketplace. It’s pretty much the only reason I still go to that godforsaken place.

At the side of the road with a FOR SALE sign that's got a hand-written phone number on it.

Not sure where you're at, but this is a US based answer.

I like Carfax. Sure they have some sponsored crap on there, but the added value of having the Carfax data right up front is helpful for me. Other than that, I also like truecar because it gives you geographic pricing comparisons based on previously sold vehicles.

FB marketplace or a classifieds site is where you will find the best deals, it's the only reason I still have an account with meta. It's almost always a better deal for both parties buying and selling person to person. Most people will be more honest about the condition of the car than any used dealers and they will also make more selling it directly than trading in or selling to a dealer. Buyers also save by cutting out the middle men. I'm into cars as a hobby so I'm fairly constantly buying and selling them, marketplace is where I do almost all of my transactions ever since Craigslist died.

Check out local dealer's websites they often try to compete to keep their Internet listings lower than the "other guys".

Craigslist.

I used cars.com to search other listings. After narrowing down the vehicle I wanted after doing test drives at local dealers, searching on cars.com directed me to a local dealer that I hadn't known before that had a great deal on the car model I was looking for.

Cargurus.com

I just found visor.vin a few days ago from a story about a cheap Ionic 6 SEL.

Autotrader has a decent filter system. Although, you'll mostly get dealer listed used cars not private sellers.

• https://www.autoscout24.com/
• https://www.marktplaats.nl/
• https://www.autotrader.nl/

I buy certified pre-owned from the dealership. Got a pretty good deal on my Audi and it came with a warranty. I think I saw the listing on Autotrader.

If you have a CarMax nearby, their lots will let you check inside their various vehicles without a salesman hovering over you. CarMax prices are high but they are no-haggle.