I've purchased 4 machines on Ebay.
First was a singer, many years ago. I think even before feedback was a way of keeping sellers honest. Seller didn't package well and it came destroyed. I then purchased another Singer. It actually came from the UK...was packaged very well and suffered no damage.
My third was an Elna. Loved the machine, but wasn't in the "Excellent" condition the seller claimed. Besides not being in the excellent condition claimed, the batteries had been left in it and leaked battery acid all on the insides. Seller gave me a little bit of a hard time, but eventually refunded my money, but didn't refund the shipping, which is the usual practice, but was quite a bit being a heavy item. I'm sure the seller didn't know much about the machine and had no idea there were batteries hiding underneath it. The 4th is my current Bernina. Seller didn't know much about the machine, and the motherboard needed to be replaced. They would have taken it back, but my Bernina dealer kindly replaced the $1000 part, saying it was still under warrenty. I don't think a dealer would do that now.
I would say to look at the feedback. I usually look at the items they have sold in the past...are they as valuable and were they packaged well? Ask questions. What do they know about the machine? If you are buying from the owner you have a better chance of knowing what you are getting.