I think that as long as you stay away from factory serviced or refurbished type dealers, you'll be ok...ebay and paypal will cover you if you receive an item and it's not what the seller said it was or not working properly. Just read the item description very carefully!! If it says "I don't know anything about sewing machines and haven't tested it"...or anything similar...run...that's the same as "buying as is" and you won't get much help in returning it if it doesn't work!
Just 1 other thing...if you do buy a machine and it's been in the cold, through the mail, etc...unpack it and let it get to room temp before you turn it on! The computers in today's sewing machine do not like to be frozen, then turned right on...good luck...i hope you get your machine!!