i love this idea
because the more they r used, the more things we can buy
But how do we stop people from killing them?
Simple... you would keep them in a 1x1 area anyways, so could also place a pressure plate under the villager. As soon as he gets killed, the pressure plate activates (because not pressed anymore) and a connected dispenser (with one inverter inbetween) shoots a villager spawn egg out to the place, so a new villager comes up.
This way you automatically get a new villager. The dispensers would be locked with a password which only the Admins and Mod+ now, so they can refill them from time to time.
Killing all villagers should be impossible due to the ammount of stored eggs (each dispenser can take up to 9x64 eggs) and the business of the place, so someone who kills one would get caught fast.