I've only bought one machine on ebay and it was a great purchase. OTOH, lots of people selling machines on ebay know nothing about sewing machines, so can't or won't tell you if it's missing parts, etc. It's just like any other ebay purchase - you have to know what to look for, check the seller's reputation, and don't pay more than you would for an item that needs some attention if you can't verify the other stuff.
Buyer feedback is best, and you can check to see what they've been selling. If they don't normally deal in sewing machines, take that into account. If they have included lots of pictures, you can check for missing parts, frayed electrical cords, dings and scratches, gunk in the works, etc.
You can also ask the seller questions, but you have to be careful. Questions and answers are posted on the listing. If you see a real bargain and ask a question that gets a favorable answer, you may encourage other bidders to jump in and run up the bid.
Darren