I just bought one on Ebay. I found that there are those who sell featherweights as a business. They start at $350.00 At that price you can expect the machine to run, and work well from the beginning. I would also expect the black case, and some attachments. Those you buy on Ebay, GoodWill, etc. more than likely are not promised to run. With a good cleaning and possibly some parts they should run well. The lady who had a smelly black case, well, she isn't alone. There is a felt pad that is under the sewing machine. 50 years of oil, etc. gets collected down there. It seems often to spill or smell up the machine. Then there are water issues that could have caused the mold. For the most part you can clean out the inside and put it open in the sunshine until it doesn't smell bad any more. Nova Montgomery has classes for featherweight maintenance that is excellent. She also sells parts for the machines, along with instructions on how to clean and maintain.
The one I bought off ebay sold for $149.00. However, the man who sold it liked to monkey around with things like sewing machines. It was pretty clean, but I don't think he knew what he was doing. My sewing machine wouldn't run. My machine had aluminum wiring, it was made in Scotland. You can find out where it was made and when by the number. Aluminum wiring is not good wiring. I had to replace my wiring for the foot control. I needed a new belt. New feet, bobbins, light bulb, and so on. That was an additional $65.00(plus postage to and from and then another one). I finally resolved the problem with what was wrong with my machine, but I was on the fence as to having to take it to a sewing machine repair guy for quite a while. So if you buy one that is not guaranteed, as being clean and running, you need to know something about how to get it there! Either you pay a pro to fix it for you, or you do it yourself.