For a modern computerized machine, I would gladly buy a used one from dealer that offers a service contract or at least one year warranty even if that cost more than buying one from a private party. Yes, most dealers have used systems that typically come in as trade-ins.
Some deals on Craig's List I'd feel comfortable with, but don't know how I'd feel about an auction site buy.
A little over a year ago, we mailed a modern Bernina 820 from Arizona to Washington, UPS or other options might have been better but between buying an actual box from the local Bernina dealer including all the foam packaging inserts and the cost of shipping plus insurance was about $200.