To be honest, I wouldn't pay $50 for that machine in that condition. I think at $60 they are asking way too much. It's going to take both time and money to restore it.
Oops! Just took a look at the back of the machine. It's missing parts *and* the back looks like it is either full of mold or rusty. Probably the only valuable part of the whole setup is the treadle; the machine head looks to be in terrible condition, and you said the cabinet is in bad condition too.