Well there is inflation, probably more like 2-3k is the new $500 beater car. And inflation hasn't kept up with minimum wage. Plus I think with cars that have airbags, ABS, backup cameras, side impact and other safety measures, the old $500 beaters didn't really have much of that so it was of course cheaper to make back then too.I remember the late 60's when a knew a grocery bagger that earned enough to buy a new $3k Plymouth Roadrunner. I bought a 70 Toyota corolla for $2400 which wasn't much cheaper, but a fun car. I was noticing the failing of US cars in especially rust control, and have always done something different. I had a nice 97 Dodge stratus with 94k miles, but in great condition and sold in about 2010 for only $2600. The $500 beater that runs for years is done for sure.
Also there's still Craigslist. Plus even though most are dealers, you can still use the major sites like Cars.com, Autotrader, Cargurus etc. Plus some of the large used car dealer lots used to have pretty competitive prices with private sales, but not sure if that's true anymore.