I have bought many bikes, and some cars and other things too, without hearing them run. Mostly turns out ok, but then consider that when making an offer. Talk to the seller and get a feel for what he is saying. If the motor is seized or has a knock and he is lying to you and saying, 'oh, just needs a tuneup and fresh gas' you will know.
I sure as heck would not pay 600 for it. I don't think I would pay that much for that bike even if it ran mint.