Make it a limit of 10 or 20 if we're really feeling an itch to put limits, but this game needs a dang premium underground gold shop like I've been nagging about for a decade.
The reality is, the economy really can't survive on anything above 5.
The only you could make it so, is if you made the gold sinks even higher.
This means that if the MO limit was set to '10', all of the gold sinks (NPC Prices, Auction Tax etc.) would need to be doubled to compensate - and I don't think anybody wants that.
I edited my comment above but I said that another issue is that most of our whales are gone and the AP card buying thing hurt more than it helped.