How to Avoid Being Ripped off by Villagers in Minecraft

Basic Knowledge

Know your Villager. The Villagers that currently roam their villages are librarians, priests, farmers, blacksmiths, butchers/tailors and generics (modified in). Farmers wear brown, librarians wear white, priests wear dark pink, blacksmiths wear a black apron, butchers/tailors wear a white apron, and generics wear green robes. Green Villagers are also called "Nitwits", and they only offer to trade after they have been turned into a Zombie Villager and then turned back to a regular villager. You can't unlock more trades by trading with them. However, due to how difficult it is to cure a zombie villager, it's best to not trade with them at all.

Know what you want to trade. People might often just run about trying to trade whatever willy-nilly, but that won't help. If you have some paper or books you wrote, trade with a librarian! If you want something enchanted, trade with a priest (though it is usually much more efficient to enchant things yourself).

Find emeralds. Emeralds are used as a currency for the villagers, and every trade is an exchange of an item for an emerald. They are found in Extreme Hills and Edge biomes between height levels 32 and 4. However, even when mining in an extreme hills biome they are rarer than diamonds, so it's best to just trade for them. If hoarded, emeralds can buy extremely rare or difficult to make items like saddles, enchanted books, and diamond armor, but the villagers that sell these are rare and most try to sell common items like bread or glass.

Check for a ripoff. These can easily be mistaken by people who trade constantly, but it is true. They can try to trade a flint and steel in exchange for an emerald, an iron shovel for an emerald, four emeralds for a diamond etc. The vast majority of a villager's trades will be ripoffs, and they will charge very large amounts of emeralds for desirable items like diamond gear and chain armor and enchanted books.

Maximizing Emerald Profits Strategically

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

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.

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

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.

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.

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.