Hey there! Good questions!
It all kind of depends on what you’re looking for. If you want to buy a nearly-new car, meaning around 2-4 years old, then most of them will be for sale at dealerships, so buying one at a dealership is likely the best course of action (because they’ll have the most selection). That also means using CarGurus, Autotrader, Edmunds, etc. to find the car is probably your best bet. You might also try Carvana for this market segment.
The pricing guides – KBB, Edmunds, NADA – all work pretty much the same way, and they are good for getting a ballpark figure on values, particularly for near-new (or actually new) cars. But none of them is gospel. In some cases, the individual car you’re looking at could easily be worth more or less than they say, depending on its history, color, features, location, etc.
Alternatively, if you’re interested in buying a somewhat older car, say 5+ years old (which most of us at Klipnik would recommend, mainly because you start saving a substantial amount in depreciation at that point), then I’d advise you against going to a dealership generally (more on that HERE). Most of the best older cars will be offered by private party sellers.
For private party sales, craigslist is usually your best bet for finding listings. Sometimes eBay has good ones, so it’s worth checking there, too.
The pricing guides for older cars aren’t very good. They tend to under price the really nice examples (which is what you ideally want to find) because they mainly use auction data as their basis, and older cars that go to auction are often not in great shape.
Hope that helps a bit. Please chime back in with any follow up questions that pop up!