Did pretty much the same on my server, day 3 or 4 MIN+ARM 50, so I controlled DS nugget trade. But instead of selling em for a price like yours (which is market 39k if I didnt do the math wrong), I sold em tiered, 5x17k, 3x20k, 1x30k each time(so most times at 17k). I didnt get banned since the total sum I made within my monopoly week was about 3,5m.

So yeah being too greedy can hurt ya! :3