I agree with PDQuilter, a dealer is the best place to purchase a used long arm. Not only can you rely on their service, but their knowledge as well. Most dealers that sell new machines also sell preowned. You just have to be ready to buy when what you want becomes available.
I'm not an EBAY user but I was looking for something for my Handiquilter yesterday and saw several on there. I don't know a thing about using EBAY however.
I also noticed on the HQ web forum there are people who list them. Probably the same for all brands. I know APQS has a sale on used ones regularly.