A $500 plane ticket is money well spent to prevent you from buying a $25k boat anchor.
I think it depends on what it is and how much of an investment it will be for you. In my experience most of the major manufacturers have kept up with good records on any machine they made, and a call to them is a good place to start as they can tell you things like who originally purchased it, any service calls they made, etc. I recently had a call to the manufacturer of a 5-axis waterjet we were looking at, and not only did they talk to me about the machine for probably 30 plus minutes, but he even called back the next day with more info.
As far as paying someone to inspect the machine I think it would cost you less to take two days and fly out yourself. Not to mention the person inspecting it knows they will never see you or the machine again, and I don't imagine anyone warrantying their info for you.
If small money for something seemingly dirt cheap on the other hand then sometimes you just have to roll the dice and expect the worst/hope the best.
