"I just see so many advertisements on this (and other sites) that say original miles of 99,999 miles or less. When all of the cars are at least 50 years old, it seems to be at best very unlikely."
It all depends on the car. A blanket statement of "original miles" with no evidence to back it up is fairly open to scrutiny and not worth a lot. However, some cars are legitimately very low mileage and that claim is 100% valid. For example, my grandpa has a '60 DeSoto that he purchased 15 months old and 15,000 miles (a lot of miles put on by the first owner who we think used it to carpool). However, it was purchased as a second car and just never really used as such. Today, it has only 43,000 miles and it reflects that with original paint, chrome, interior, and the factory plastic seat covers. He also has fuel and maintenance logs back to day 1 of ownership, along with all receipts. Similarly, he ordered and bought a '77 New Yorker (without A/C!) and that has only 13,000 original miles for basically the same reason as the '60. It was always a second car, never taken on long trips and a door ding ended its "errand" days early on. I don't know if the car has even ever been in the rain. It also has a fuel and maintenance log and all receipts since new. It even looks and smells like a new car and, I might dare claim, could be the nicest one left in existence. Both cars get run about a half dozen times per year on the most perfect weather days, about 15-20 miles round trip, including a short stint on the freeway. That has been the pattern for most of my life with a few car shows sprinkled in between.
My '66 Fury was purchased from the original owner. Just like with my grandpa, he had a fuel and maintenance log since day 1, as well as oil change stickers and receipts, which backed up the 49,800 miles it had when I purchased it. As one of only two new cars he ever purchased, it was never used in the winter and mostly saw some highway miles for weekend trips within the state. My '59 DeSoto was purchased by an older gentleman new and I believe was rarely driven. In 1986, an ad for the car states that it only had 16,000 miles, always garage-kept, and with the original tires. I purchased it from my friend who bought it from that ad in 1986 and know that the 40,000 miles I purchased it with are legitimate.
Circumstances are different for everyone and sometimes cars just aren't driven much or perhaps were parked early in life and later revived. Cars that have good documentation and low ownership are often easy to verify. Cars that are sold frequently and lose documentation, not so much.
Only trouble is that where am it I can be an hour or two before you encounter oncoming vehicle.... Still a fun feature though 
