Villager trading has many advantages. It allows you to:

 Obtain exclusive resources like explorer maps and globe banner pattern.

 Obtain rare items like saddles and name tags.

 Obtain items that are hard to get loads of otherwise, like apples.

 Obtain renewable diamond equipment without having to mine for diamonds.

 Obtain valuable enchantments through books, including treasure enchantments such as mending.

 Make otherwise non-renewable resources renewable, like glass and laps lazily.

 Obtain infinite amounts of late-game food, such as golden carrots and cooked steak.

 Make otherwise useless items like rotten flesh useful because they can be traded for emeralds.

 Get a lot of experience while trading, for enchanting or tool mending.

 Notes

 Before you proceed, here are some things you'll have to do:

    For a villager to offer useful items, you must level it up by trading. The ideal trades to do this are listed below, but as a rule of thumb, selling items for Emeralds increases a villager's experience more than buying items, and later trades offer more experience.

  The items that villagers offer are random. If you get offers you don't want, you should either break and replace the job site block if you haven't traded with the villager yet, or you need a way to efficiently kill off villagers without lowering popularity. The best way to do this is lava, or drowning if you are using flammable blocks.

 How to exploit villagers

 Zombifying and curing

 If you zombify a villager and cure it, preferably multiple times, you can reduce the prices and make the trades easier to do. If you cure the villager enough times, you can make all the trades cost one emerald or one item. This table shows the # of times you need to cure the villager to max out all the trades (1 emerald or 1 item). Works for both Java and Bedrock.

 Crazy villager exploits

 This video is in Bedrock edition, but most of it works in Java edition. The first part describes a bug in zombie villagers, which is Bedrock Edition only. Skip to 4:28 for the actual villager exploits that work for both editions.

 Below are a few broken villager trade loops:

   If a librarian has a maxed out bookshelf trade and a book trade, buy a bookshelf and break it without silk touch. Sell the books back to the librarian. You get 3 emeralds, subtract the cost of the bookshelf, and you get 2 free emeralds.

   If a Fletcher has the maxed out gravel to flint trade and flint to emerald trade, start with as many emeralds and the same gravel amount as the emerald amount. Give the villager an emerald and a piece of gravel for 10 flint, and sell the flint back to the villager. You get 9 emeralds each time you do this, but it costs one gravel, which you can easily get from under the ocean, mining, or in gravelly mountains. You can also sell the flint to leather workers, tool smiths, and weapon smiths, as long as you maxed them out.

   If a fisherman has the maxed out bucket of cod trade and the cod to emerald trade, start with 1 emerald. Buy a bucket of cod, pour it, and kill the cod. Sell the cod back to the villager. You don't get free emeralds, but you get a free bucket each time, which you can reuse for dripstone lava farms in 1.17 or for storing lots of powder snow, water, milk, fish, axolotl, and lava.

   This trade loop requires a cartographer with the maxed out glass pane trade and a librarian with the maxed out glass trade. Start with 3 emeralds and buy 12 glass from the librarian. Convert the glass to glass panes. You should get 32. Sell them to the cartographer for 32 emeralds. Subtract the 3 emeralds to buy the glass, and you get 29 free emeralds. This trade loop is similar to the one above, but uses a cleric with the maxed out glass bottles trade. You still need a librarian with the maxed out glass trade. Start with 3 emeralds, buy 12 glass from the librarian. Convert the glass to glass bottles. You should have 12. Sell them for 12 emeralds. Subtract the 3 emeralds, and you get 9 free emeralds. This works better than the cartographer because the cartographer locks the trades before you finish selling the glass panes. If you have many cartographers, use the glass pane trade loop. If you don't have many cartographers, use the cleric trade loop.

   This trade loop doesn't give you extra emeralds, but cooks your food for you, and you get more cooked food than you would get by smelting it. You need a maxed out butcher with a raw meat trade and the corresponding cooked meat trade. You can only use pork chop or chicken. You need a piece of meat of your choice. Sell the meat to the butcher and use the emerald to buy the corresponding cooked meat. If you used pork chop, you get 5 cooked pork chops, which is 4 more than you would get from smelting. If you used chicken, you get 8 cooked chickens, which is 7 more than you would get from smelting. You do not get any free emeralds, but you get 4 extra cooked pork chop or 7 extra cooked chicken. This doesn't work with fisherman because you need to pay an extra fish to get cooked fish.

   This trade loop requires a wandering trader with a two tall flower trade and a shepherd with the maxed out corresponding dye trade. Buy a two tall flower and craft it into two dye. Sell the two dye for 2 emeralds. You get one extra emerald. Another similar trade loop is to buy the dye directly from the wandering trader and sell it to the shepherd. You get 3 emeralds, minus the cost of the dye, and it is 2 extra emeralds. You can also buy 3 ink sacs and sell them to a librarian for 3 emeralds, minus the cost of the ink, and it is 2 extra emeralds.