I make purchases on EBay all the time, am usually satisfied but have run into a few problems, most of which were resolved. That said, I probably would not buy a new high-end sewing machine on EBay.
When I purchased my Bernina 440QE, I bought at a local shop with an excellent service reputation, and I purchased an additional warranty. I can take my machine in at any time for cleaning, repair, etc. without charge, and that means a lot to me. I also got free lessons on using the machine. With the newer machines, there are often software updates that can be loaded in when the machine is in for warranty service.