Not everyone wants to repair machines . I don't care if it works or not, but most people (and sounds like the OP is one since she's new), want a machine that actually sews. If she is paying a premium price, she should know it will do what she needs. If the owner can't prove it's operational, and she can't sew with it...that's a big piece of info. she should know.
Parts and accessories like a foot control may cost as much as the expense of buying it. Berninas don't come cheap and neither do the parts. If it needs any sort of repair test driving it would be the way to go.