What he's talking about is during Cata, there was a gigantic influx of the epic gems that you would get from running DS. It was the only place you could get them, and a mere couple of weeks after the instance was released, you would see massive stacks being sold at cut-rate prices. It stuck out like a sore thumb, and the cause was duping. There was no feasible way to acquire that many of the gems and still be able to sell them at the prices they were selling. In some cases, there wasn't even the possibility of acquiring as many gems as were being sold at one time.