Welcome to the oRg. There are many things to look at when considering the purchase of a used machine. First, check to see if there are any maintenance records, how often was the oil changed. How good are the tires? Chain and sprockets, do they look worn? Does the chain look neglected, like no lube or rust? Does the engine smoke? How does it shift? Most owners will not let you test ride one without cash in hand, so you need to work this out somehow. Check the bar ends and tips of the mirrors, tale tale signs of tip over or road rash. Check the over all cleanliness of the bike, swing arm, engine, wheels, plastics, etc. Do all the lights work? What mods have been done? If so, are the original parts available and come with the bike? Have you checked to see what insurance will cost you? Dont ride without it, a slow speed fall can cost you thousands. Does the owner have the the hump, rear seat, and grab bar? Check to see if the fork seals are leaking?
Also, after the purchase of a used bike, go change the oil, and inspect and/or replace the brake pads and clutch/brake fluid. Thats all I can think of right now. Do you have any pics?