If you're going in person, waste a roll of film through it. Just to make sure it can wind/advance properly. Once it's done with the film, fire it with the back open looking through the taking lens to see that it's shutter is working right. The main problems you are apt to have is shutter being off / unreliable, film winding not working right, and physical lens damage such as scratches or fungus damage.

If you get a good working yashica for $45, excellent. I had someone give me a yashica-C, then I spent $100 on a CLA to make it work like new, and have shot a bunch of really nice photos with it despite it not having the most desirable lens option. Still a good deal in my book.

A cheap rolleiflex would be the automats with the 75mm tessar f3.5 lens. I bought one for $200 and it's excellent. The big money rolleiflexes have mostly planar lenses.