Other people have made some excellent suggestions.

The key is to think "real" world.

If the PC's are doing this what's going through the heads of the rules, other powerful NPC or the common people?

You've had the excellent suggestion that the wealth of the population would be so much greater than other places that you'd get an influx of people and crime would sky rocket. Jealous neighbours might invade or local rulers may object and take action against the PCs.

The trick as the DM is to consider the repercussions of the PC's actions and ensure the world they're in behaves appropriately. It sounds like you've lost control so take the suggestions you have received so far an implement them.

Cheers
Grrosgor