Provided the machine has been cared for and serviced properly, the mileage, if anything, is low for the price being asked. If the extended warrantee still applies to you, that should give you further peace of mind. Speaking purely for myself, I have 18,000 miles on my Burgie and it's just beginning to warm up! Check the scooter over carefully. If it looks as if it has been cared for ie. no scratches or dings and the owner seems a reasonable person, I think you have a pretty good deal there. You could, of course offer him/her less initially, who knows, you may get lucky!
Good luck!
Bob