The original owner helped me do some research and suggested I try spinning the ring around several times before holding it in place while autofocusing to see if the lens still focuses as it should under normal conditions.
And it does!
CharlieB I'm sure you are right in that sense of the problem. The owner had not used the lens for a long time and it had been sitting in the bottom of his camera bag. The more I spin the ring and have the lens focus, the less the motor pulls on it. So it turns out that it's just a bit stiff. Coupled with the lens being overheated in the car though, it did seem like a very serious problem.
I talked to someone once who kept their camera equipment in a cooler in their car. Now I know I didn't realize what effect heat could have on a lens.
And thank you for the tip Mt Spokane Photography. I'll start considering repair cost when buying used to see if I'd actually be saving any money.
Lenses getting hot in the trunk are a nightmare. I am able to repair my own lenses to some extent (repaired two old Minolta MC lenses Sunday). Both had the lube separated likely from heat, and run into the aperture.
Sometimes, the lube in old lenses just hardens, particularly if they have not been used for 10-15 years. Then it has to be scraped out and replaced.
Not all old lenses are easily cleaned, but the Minoltas can be done in 15 minutes, and ten minutes is spent swabbing the lube off the aperture blades.