Also, be careful of how you get information from the seller, so as not to "prompt" the answer you want to hear, if you know what I mean. For example, don't ask "How often have you oiled it?". Instead, do your research first and know the answer to the question of how often it should have been oiled (best is one drop of oil every bobbin change, for almost any machine, long arm, or domestic), and act dumb and ask "How often should I oil this if I buy it?"
You are more likely to get the truth that way. And the truth is really important on such a large purchase.