I must have fallen asleep and somehow I woke up in 1994.
In 2019, which is the year it was when I fell asleep, the internet had become totally pervasive to the point where detailed objective information was available on just about any subject. And we were able to get to all of it on our phones if we wanted to!
If I were looking for a preowned car in 2019, I would have gone to a few websites like Edmunds, TrueCar, or KBB. Those listed the values for used vehicles, even adjusted for mileage and options. Then, I would have gone to a site like Autotrader to see how many of the vehicle I wanted were out there on the market, and what people were asking for them.
I suppose I'm not sure what people used to do. I bought my first car in 1986, it was a complete guessing game then.