Villager trades and economy

Villagers offer extremely useful trades and are exploited for this...but how did they get them? How did the armorer get diamonds to make the armor?

I think that Villagers should make an economy of their own: every working Villagers would have a second inventory for items they buy, and they will give those items to Villagers who need them: also a Villagers will be able to sell an item only if he has the ingredient so make it: this would mean that the armor won't sell Diamond armor piece unless he effectively has diamonds to craft it. Or for example, we won't be able to buy a bell unless the Villagers has gold, which he gets from  a cleric