First impressions can be important. Is it an un-loved mongrel, skulking in the corner of a shed, or worse still, outside; or is it something that the PO has obviuosly lavished lots of time, effort and $$ on? These questions will probably be answered by the amount being asked.....unless it is an early 750, in which case all bets are off.
If it doesn't run then you have to ask yourself the big question as to whether you want to try and make it run. If it does run does it sound healthy and not smoke?
Assuming it is a machine that has been in regular use look specifically at the consumables: tyres, chain/sprockets, battery, sterring head bearings, swing arm bearings etc etc. All of these things, if needed, can make a $500 gem become a $2000 nightmare! And if the seller is asking a premium, deduct value for consumables that need to be replaced.
Hereabouts a shop which will look at a bike for you will charge about an hours labour. Bear in mind, though, that the mechanics will probably have never seen a bike like ours, so may not be able to advise on specifics applicable to the model.