Try scanning ads on this website to see what similar setups have sold for (asking price is much less relevant):
http://www.longarmuniversity.com/machines_for_sale.htm
It would at least give you a ballpark.
I don't know if this website puts "sold" on machines, but you can see what people are asking:
http://www.houseofhanson.com/formerlyowned.html
I wouldn't be afraid of buying too much machine.
Edit: I do think you need to ask why the person is selling. Is it to upgrade? Or is she not using the machine. If not using, ask why.