So I went for this one.
I was handed the original 2001 green log book with one owners name on it, 19900 miles on the clock.
As the guy was about to start counting the cash I decided to warm the bike up. Bleeeeuuuu, dead battery. So off he went and bought a new battery. (£43)..fool.
In his garage without electric he starts putting the new battery in. I hear him saying, "Oh, I'm putting this in the wrong way round" and corrected his mistake. I asked him if he connected the cables and he said no. So put key in and nothing. Oh ffs. Pulled every fuse and they were fine. He thought he had a dud battery.
By this time it was late so I went home and googled cbr reverse polarity and found that the main fuse lived alone away from the other fuses and was easy to miss. I was 99% sure this was the fix. I phoned him and said I'd found the problem and meet him tomorrow.
The next day he rings and says the battery was a confirmed dud!
Anyhoo, got to his garage after many postponements and stuff and he had the battery fitted the correct way. I lifted the seat, changed the fuse and put the key in and she lit up. Fuse was well blown.
Well, I got a new battery and he gave me 20 notes for petrol.