I bought two machines at yard sales. The first one was a Sears Kenmore and it sews wonderfully. The machine is about 30 years old and if it ever breaks no parts are available, but for $10 you can't go wrong.
The second machine I bought, was right next door to the first one I bought. (community yard sale) It is a Singer and I got it for $15. It worked great but the bobbin encasement was loose. I got it fixed and serviced for $59 and it now sews wonderfully.
You have to be careful at yard sales and garage sales, but I was very fortunate. Look them over very carefully and if you can ask them to plug it in and demostrate it works. Some folks will gladly do that for you. Good luck to you.