I agree with the others that this is a 550K, not an F bike....the rear peg mounts (ears) say so as well as the tank. If it started cold on the third kick, that's probably a good sign. That said, it doesn't mean that regular maintenance has been performed over the years. I would ask him if he has adjusted the cam chain and, if so, did it adjust. In my experience, the 550's tend to have tensioner issues, especially if that service was not performed at recommended intervals or if it sat for a long time.
Just general stuff to check...How does it shift? Is there rust in the tank? Has the wiring been messed with? Condition of chain/sprockets? Steering head bearings (does it "catch" in the center when turning the bars?)? Wheel bearings? These are all things that can add up $$ quickly if they need replaced. I wouldn't necessarily worry about a minor oil leak as many bikes this age develop leaks....unless it just bothers you or leaks alot.
My '74 550K was in fairly comparable condition when I got it a couple of years ago and I think I paid $600 for it....but had to drive across state lines to get it. I rarely see any CB listed for under $1000 unless it doesn't run or needs major work. I agree with Flybox1.... maybe offer somewhere around $500 to start, but my comfort zone would be to pay up to maybe $900? That's just me, though and I'm in Indiana (not familiar with your market). This bike does seem to have low mileage going for it, though....that's a plus.
Good luck!