I bought an older Bernina at an estate sale. I had owned one so had a bobbin and casing to take with me. I also took needles, thread and fabric to make sure it sewed well. A lot of sellers just turn it one and press the foot pedal and say it works. As Cathy said, not always so. If I'm paying more than a few bucks for it, I want to make sure it works.
I have bought a couple machines at garage sales for $20 without testing, but they were old Singers that appeared to be in good shape. One I mostly bought for the case which would fit my 501 that I didn't have a case for. It's a 401 and I still haven't tried it to see if it does sew. I also bought a 185J in a table that I really bought because it was only $20 and was so cute and little and GREEN