Condition means a lot for price. A machine with intact decals and not a lot of scratches from pins will be worth more. Avoid those that have tape on the bed as a seam allowance guide; you won't be able to get the tape off without damaging the finish.
It is *very* important that the seller understands how to pack the machine for shipping. Many machines are damaged because of improper packing. Knowledgeable sellers will describe their packing technique in the listing. If at all unsure about packing, email the seller before bidding. Machine and case should be shipped in separate packages. Double boxing is good.
Also, pay for insurance (most sellers do not provide insurance) so you can recover from any damage in shipping.