I bought my Brother Duetta 4500D used off of ebay for $2400. I know it was a big risk, but I emailed back and forth with the original owner who happened to be Project Runway Designer Jillian Lewis and I felt like she was being honest. I also spoke with her assistant on the phone to make sure it really was her I was talking to.
Anyway, I could never afford or justify a new machine at this level (my local shop wanted $8000

), so I took the risk. I did get about $200 cash back through Bing cash back which was a reward program that was available at the time, but then I spent $200 to have it serviced. Regardless, I have been very happy with my purchase and I use it all the time. It had a lot of sewing use, but had almost zero embroidery use from the original owner. Just wanted to share a success story with buying a used machine. And if you can get one through a shop where you can actually see the machine and they know the original owner the risk is cut down considerably from what I did.
Would the shop provide any kind of warranty on a used machine (like 30 days or something?)