As others mentioned, make sure the bobbin case is there, it's about a $45 touch if it's not, and replacements often don't work like the originals. Some people like to steal them unfortunately.
In my opinion, things like belts and power cords and other consumables wouldn't put me off a machine, they're a negotiation point though (I was hoping to sew with it tonight... but it needs.... ) The important parts are the non-replaceables and the expensive to replace parts (body condition - paint, bobbin wider bracket not broken off, motor, etc) , even a broken bobbin winder is not the end of a sale, as long as the bracket is intact. There are parts for these machines all over, and a few really reputable people to get them from on the internet.
That said, I like to "force appreciate" the value of my machines: buy dirty, grungy, machines that haven't been maintained for less money, and raise the value of them by fixing them up both under the hood and body wise. If you just want to sew with little work, look at everything you can.
If it doesn't sew, check the #1 most common reason that they won't:
There's a locating finger for the hook that becomes dislodged when clearing a thread jam. See this link for a visual of it:
http://thefeatherweight221factory.co...MachineJam.php - The last 3 photos go through what I'm babbling about. 2 of the 4 machines I've bought have had this problem, including the 222 I scored 2 weeks ago. Now that you know how to fix it, it's also a negotiating point. There's not a lot that goes permanently wrong with a featherweight.
The other thing, adding to what csharp said, if you're going to check ebay, there's a way to do an advanced search. Look for -sold- machines. Completed auctions, etc. This will give you a proper idea of the value of a machine. The asking prices are out to lunch in a lot cases, and can cause you to pay more than you should for a machine.