I used to have a regular customer when I was selling records, looked a bit like Trigger from OFAH who would always buy 10-20 singles off me every week.
Even though I rarely charged more than 50p he would always bargain me down (accepted as I needed the money).
Shocked to walk in this local record shop one day and saw him behind the counter and found most of my old singles selling for about 10 times what he had paid me.
Tried to engage with him, but he didn't want to know.
Can't bring myself to ever go back there even though there aren't many record shops in my area.