If you want a PC ran economy, I think that you can control more with prices than with hard limits.

For an example from RL: Let's say I can sell a Mt. Dew for $1. I'll sell a lot of Mt. Dews if my costs are below $1 each. Now let's say everyone is making Dew or a substitution that's just as good and I can only sell it for $.50 each? My costs are $.75 each. I'm not going to make any more.

Now, same thing applies in a game. If you want make some supply/demand code that says if too many of item x is sold to a NPC shopkeeper (or all shopkeepers in the area) the price will drop to a point where it's no longer profitable. You get the result you want without a 'you can't do this' artificial cap.

This'll also benefit other players. If there's a glut on the market, players can buy it for less than if there were a shortage. And then the items that there are shortages for would be produced because that's where the money is. Thus prices balance out over the long term. Not only that, but if businessmen/capitalists exist in your environment, they get to partake in something very important: Profit.

Profit then means they can become more efficient (train or buy upgraded equipment), they could hire other people to RP with... profit is beautiful, and with the proper price restraints your problems should be solved.

Also, you can make NPCs buy stuff at a lower rate and sell at a higher rate. That way PCs can easily undercut them and be preferable if RP is your objective. People respond to incentives, after all.

Of course, translating an economy to a RPI isn't a simple exercise. :>