Here is a link to completed listings for this machine on eBay. The green prices indicate the price it actually sold for. Black prices are listing prices, not sold prices.
http://www.ebay.com/sch/i.html?_from...p2045573.m1684
Click on the different listings to get more details. I notice some machines sold for around $300 while others sold for up to $600 or so. Usually there are differences in condition, accessories, etc. although sometimes it is just a matter of chance as well.
Based on my glance at these listings, I would think that $550 may be too much for this machine unless the seller actually gets it serviced first. A machine that has sat around unused for a long time, even if it was never used much, can develop problems and you need a technician to make sure that the sealed gears are still greased, etc. Prices for service checks vary from one area to another. In some places you can get this done for under $100; in others they won't even look at a machine for $150. If you pay $550 and the machine turns out to need servicing, you could be out another $100 or more.
Do you know the difference between this machine and the Brother PQ1500SL? Because you can get a new Brother PQ1500SL for $650 on Amazon:
http://www.amazon.com/Brother-PQ1500...dp/B0149GG9V4/
Used machines typically sell for around half their original value -- a little more for a mint condition, newer machine with lots of accessories -- a little less for other machines. It just seems to me that $550 is a bit high.
Edit: I did find this review which indicates it's a decent machine.
http://www.badassquilterssociety.com...achine-review/