With 12k+ miles on it my AN650 runs great - even better than when new. My last Japanese motorcycle saw 66,000 miles before I sold it - and ran perfectly at that point. That is very typical of Japanese bikes that have been properly maintained. If they owner can show proof of maintenance, I would not hesitate to buy it. Without proof of maintenance, I'd pass. Be aware that the AN650 should have the valves checked at 14,500 miles. If that was not done, but oil was changed, air filter & spark plugs replaced, etc - I would get the cost for that valve adjustment deducted from the price (figure 6 hours labor at the going rate in your area), and have it done right away.
It is actually much better for a motorcycle to be used regularly, than to be ridden sporadically and sit idle for long periods in between rides. And a 3 mile ride is harder on a bike than a 300 mile ride. With proof of maintenance, I'd buy a bike that has been ridden 10,000 miles a year before I'd buy one that has been ridden 1,000 miles per year.