I have purchased used machines repeatedly from
www.shopgoodwill.com for our charity quilting center at church. I've probably bought at least 15 machines. Only twice have they been damaged in shipping and the entire cost of the machine was refunded to me. I try to read the description of the machines to make sure the bobbin case, pedal and cord are included. I also limit to Kenmore, White or Singer machines. The one thing you must watch for is the shipping. You can enter the zip code of where you live and you will know exactly what you will be charged for shipping. I have gotten really good mechanical machines on this site for as little as $15 plus $15 or so in shipping. The weight of the machine is crucial to what the shipping will be, of course. Sometimes they had a $2 fee for handling which I assume is packaging materials. The two times a machine was damaged in transit, I was refunded the purchase price and was able to get the machine repaired locally, so it was a win-win situation.
I also purchased my white Featherweight on the Goodwill website two years ago. It was shipped perfectly.