Yeah they do that periodically. If you find someone 'laundering' PSN cards for 50% it can make sense, but it's still big money relative to the value of game credits. The top card, for example, nets you $12mil for what amounts to nearly a day's pay for an average labourer. That's a day's work for a few cars, or a jet and a yacht, etc. It's rather steep. This is a video game, after all.
Unfortunately, that is the reason glitches like the above are fixed in hours or less, while more significant losses occur daily to players due to unfixed glitches. If the glitches costs R* perceived revenue, it is fixed ASAP. If it costs players money; it is often not fixed at all, because R* knows that only a few folks will open tickets requesting refunds for the lost credits. It's greedy, it's frustrating, and it is the direction in which the industry has moved. I'd rather pay 5 real dollars a car, or similar. After all, a $2mil car with 500k in mods is $40USD. That is without getting into how R* actually charges you by the hour to play the game.....