My boss asked for some more advice about the missing photos and videos on his son's MacBook Air. I recommended he search for the programs that recover files. He found a few, and DiskDrill looked the best.
To help out, I installed it on my MacBook Air (which coincidentally I had acquired from my boss' other son) and let it search for files. It found files.
I would like to note that the other son formatted the hard disk before it came to me. I did a fresh install of Mac OS X on it. Months later, DiskDrill found about 30,000 photos there and about 2,000 videos. I saved a few of each just to check them, and sure enough, they were personal photos and videos from the other son. Nothing untoward, just happy snaps.
So I learnt a good lesson. Formatting your hard disk when you sell your MacBook is not a safe bet as many of the files can be recovered by the purchaser. I will investigate more drastic format-and-overwrite techniques before I sell anything else.