How to Avoid Being Ripped off by Villagers in Minecraft - Maximize emerald profits strategically

This method will not work after 1.7 because villager breeding and trading mechanics will be greatly modified.

1. Find a villager that trades for a mass-producible resource like paper, written books, or wheat.

2. Trap the villager in a well-lit hole or house. That way, you won't have to find it again and to prevent it from being killed by zombies.

3. Plant/breed the resources and harvest them later (or harvest them from a preexisting farm). Make sure to harvest only enough for seven trades.

4. Open up the place where you trapped the villager and trade the resources you obtained. After making a single trade seven times (seven emeralds total) the trade will instantly disappear a new one will take its place.

5. If the village has a large population and/or is well defended, kill that villager. Don't use fists, arrows, or potions, as they cause the iron golems to attack and they are very powerful and hard to kill. After you kill the villager, there will be room for a new baby villager to spawn, and it may have another useful trade.

6. Kill a fourth of the villagers, if there is a high population and they are well defended. If none of them have good trades and are not breeding, doing this will mean the remaining three quarters of the population will repopulate the lost fourth and the new villagers may have better trades.