Justin has made a good point. I would only buy used or a demonstration model if the seller guarantees it, brings it and sets it up for you.I second (or is it "third") Detlof's advice.
I also second the assertion, the order of importance (decreasing) in analogue is, TT -> arm -> cartridge. This would hold assuming you have the mechanical parts well-matched, a service offered by manufacturers & dealers: you don;t really have to worry about it, just listen & you'll know.
Just start with s/thing that's easy to use & reasonably priced & I believe you'll be happily surprised!