I am late to the party here, but definitely consider myself an opportunistic flipper.
My most recent was a Cort A6 listed as faulty for £200. I had it repaired within 30 minutes at home (new pot and switch) and then PX'd for £350 a couple of weeks later. The buyer even said in person it was worth more repaired, but they didn't want the hassle. I felt no guilt here.
I have calmed down recently, as the continual carousel is tiring (maybe I could play what I have?!), as well as the market drying up as people are more aware of what things are worth. I also know in the past certain well known shops have acquired stock privately on FB etc.