This merchant deals with all things farming.

Trade 1

10 Sweet Berries -> 1 Emerald or

10 Dried Kelp Blocks -> 1 Emerald

Trade 2

5 Pumpkins -> 1 Emerald or

4 Melons -> 1 Emerald

Trade 3

1 Emerald -> 1 Allium

1 Emerald -> 1 Azure Bluet

1 Emerald -> 1 Blue Orchid

1 Emerald -> 1 Cornflower

1 Emerald -> 1 Dandelion

1 Emerald -> 1 Lily of the Valley

1 Emerald -> 1 Orange Tulip

1 Emerald -> 1 Oxeye Daisy

1 Emerald -> 1 Pink Tulip

1 Emerald -> 1 Poppy

1 Emerald -> 1 Red Tulip

1 Emerald -> 1 White Tulip

Trade 4

1 Emerald -> 3 Black Dye

1 Emerald -> 3 Blue Dye

1 Emerald -> 3 Brown Dye

1 Emerald -> 3 Cyan Dye

1 Emerald -> 3 Gray Dye

1 Emerald -> 3 Green Dye

1 Emerald -> 3 Light Blue Dye

1 Emerald -> 3 Light Gray Dye

1 Emerald -> 3 Lime Dye

1 Emerald -> 3 Magenta Dye

1 Emerald -> 3 Orange Dye

1 Emerald -> 3 Pink Dye

1 Emerald -> 3 Purple Dye

1 Emerald -> 3 Red Dye

1 Emerald -> 3 White Dye

1 Emerald -> 3 Yellow Dye

Trade 5

5 Emeralds -> 1 Acacia Sapling

5 Emeralds -> 1 Birch Sapling

5 Emeralds -> 1 Cherry Sapling

5 Emeralds -> 1 Dark Oak Sapling

5 Emeralds -> 1 Jungle Sapling

5 Emeralds -> 1 Oak Sapling

5 Emeralds -> 1 Spruce Sapling

5 Emeralds -> 1 Mangrove Propagule

Trade 6

2 Emerald -> 1 Spore Blossom

1 Emerald -> 2 Small Dripleaf

1 Emerald -> 1 Big Dripleaf

Trade 1

20 String -> 1 Emerald

5 Ink Sac -> 1 Emerald

5 Nautilus Shell -> 1 Emerald

Trade 2

15 Cod -> 1 Emerald

13 Salmon -> 1 Emerald

6 Tropical Fish -> 1 Emerald

4 Pufferfish -> 1 Emerald

Trade 3

2 Emerald -> 5 Glow Ink Sac

Trade 4

3 Emerald -> 1 Luck Potion (5:00)

Trade 5

2 Emerald -> 1 Sea Pickle

3 Emerald -> 1 Brain Coral Block

3 Emerald -> 1 Bubble Coral Block

3 Emerald -> 1 Fire Coral Block

3 Emerald -> 1 Horn Coral Block

3 Emerald -> 1 Tube Coral Block

Trade 6

8 Emerald -> 1 Pufferfish Bucket

8 Emerald -> Cod Bucket

Trade 1

15 Coal -> 1 Emerald

22 Flint -> 1 Emerald

12 Quartz -> 1 Emerald

Trade 2

20 Stone -> 1 Emerald

16 Granite -> 1 Emerald

16 Andesite -> 1 Emerald

16 Granite -> 1 Emerald

Trade 3

1 Emerald -> 8 Sand

Trade 4

1 Emerald -> 4 Red Sand

Trade 5

1 Emerald -> 2 Pointed Dripstone

Trade 6

1 Emerald -> 1 Terracotta

Trade 7

1 Emerald -> 1 Calcite

Trade 1

4 Scute -> 1 Emerald

32 Rotten Flesh -> 1 Emerald

24 Feather -> 1 Emerald

Trade 2

5 Nautilus Shell -> 1 Emerald

22 Nether Wart -> 1 Emerald

24 Paper -> 1 Emerald

Trade 3

3 Emeralds -> 1 Experience Bottle

Trade 4

2 Emeralds -> 5 Glowstone

Trade 5

1 Emerald -> 1 Gunpowder

Trade 6

2 Emerald + 5 Arrow -> 5 Tipped Arrow

This section refers to features that are coming soon

The data format for a wandering trader variant is as follows: {Weight:1,Name:"",TradeTable:[]}

• Weight (optional) works as weight in a LootTable does, the higher the weight, the more likely that trader will be selected. If missing, the weight defaults to 1
• Name will be used when announcing that the trader has spawned ("A {name} Has Spawned!").
  
• TradeTable is a list of lists referring to each trade the merchant has. Each sub list is a list of possible trades for the wandering trader to have. The first sublist is a list of possible trades for the first trade slot, the second sublist for the second trade, and so on.

The trade format is as follows:{Weight:1,Tradedata-blocked:{}}

• Weight (optional) works the same as mentioned earlier
• TradeData (optional) contains the information for the trade to be added. This may be left blank if you want a chance of a trade not appearing.

The contents of TradeData follow that of a typical villager trade, with buy and sell values, and and optional buyB value:

buy:{id:"minecraft:emerald",Count:32},buyB:{id:"minecraft:book",Count:1},sell:{id:"minecraft:enchanted_book",Count:1,tag:{StoredEnchantments:{id:mending,lvl:1}}}}}

• buy stores the item being given in exchange for the item in sell
• buyB (optional) is a second item to exchange. (This is used in librarian trades, Emeralds + Book = Enchanted Book).

The buy, buyB, and sell tags each of 3 subtags:

• id - The string id of the item
• Count - (optional) the amount of the item. Probably defaults to 1
• tag - (optional) NBT data on the item.

This section refers to features that are coming soon

Although I tried my best to explain how modding works, it may still be confusing for some of you. This tutorial will take you though the step-by-step process of adding your own custom merchant.

For this tutorial, you must have basic knowledge on how do create a datapack. If you do not know how to create a datapack, you can find plenty online. TODO: find a good tutorial and link it here

First you want to create a new datapack with a new function. I'll call mine new_merchant.mcfunction and put it in the namespace mypack.

Inside the new_merchant.mcfunction we need to initialize the trader and set its trades:

Creating the villager function

#This command does the initial setup for the merchant, assigning a name, weight, and setting up the tradetable
data modify storage clgd.trader:data NewMerchant append value {Weight:1,Name:"Special Merchant",TradeTable:[]}
#         I recommend changing this value ^^^ to be something unique. If multiple packs use the same value, then there will be issues with one pack overwriting the other. 

#This adds the first set of trade options to the tradetable
data modify storage clgd.trader:data NewMerchant.TradeTable append value []

#This adds a trade option to the first set of trades
data modify storage clgd.trader:data NewMerchant.TradeTable[-1] append value {Tradedata-blocked:{buy:{id:"minecraft:coal",Count:48},sell:{id:"minecraft:diamond",Count:1}}}

#The 'data modify storage clgd.trader:data NewMerchant.TradeTable[-1]' means we are modifying the last set of trade options. Since there is only 1 right now, that means we are modifying the first one.

#This adds the second set of trade options to the tradetable
data modify storage clgd.trader:data NewMerchant.TradeTable append value []

#This adds a trade option to the second set of trades
data modify storage clgd.trader:data NewMerchant.TradeTable[-1] append value {Tradedata-blocked:{buy:{id:"minecraft:diamond",Count:1},sell:{id:"minecraft:emerald",Count:2}}}

#Add a second trade option to the second set of trades
data modify storage clgd.trader:data NewMerchant.TradeTable[-1] append value {Tradedata-blocked:{buy:{id:"minecraft:coal_block",Count:5},sell:{id:"minecraft:emerald",Count:2}}}

#Now that there are 2 trades, the 'data modify storage clgd.trader:data NewMerchant.TradeTable[-1]' refers to the second set of trades

This function will create the data for a new merchant with 2 trades. The first trade will always be 48 Coal -> 1 Diamond and the second trade will either be 1 Diamond -> 2 Emeralds or 5 Coal Blocks -> 1 Emerald

Running the function

Now that we've written the function to create the villager, we want to make sure that the function gets run to initialize the trades. This can be done by creating a new namespace called clgd.trader then inside that namespace create a new folder called tags, another subfolder called functions , then a file called set_default_trades.json. You'll also need to create another file called update_trades.json

So your folder structure should look like:
clgd.trader -> tags -> functions -> set_default_trades.json

The set_default_trades.json should look something like this

{
  "values": [
    "mypack:new_merchant"
  ]
}