KELP

Kelp has no dye in it and no place in a crafting grid on its own, so nobody buys it to build anything. What it is good for is food and fuel: dried and stacked into blocks it burns for a long time in a furnace, and eaten raw it is one of the few things in the game you never have to cook first.

It only grows underwater, coming off kelp plant in ocean and river biomes, so a base far from the coast turns the trader's 3-emerald offer into the difference between a swim and a walk. The trade hands over already-harvested kelp, no diving or a stack of it rotting in your inventory required.

One seller only. The Wandering Trader asks 3 emeralds for 1 kelp, and there is no offer anywhere in the game that pays them back for it.

Job villagers leave it alone entirely. Neither the Fisherman nor any other profession lists kelp on their trade table, in either direction, so the trader is the whole market for it.

He asks 3 emeralds each, and 58 of the other 75 come cheaper. Price is the easy part: he draws his stock from a shared pool, so catching one that happens to be carrying kelp is the real constraint.

The Wandering Trader asks 3 emeralds for kelp. 8 of the 72 villagers that pay emeralds, emptied out:

3 lava buckets against 90 flint for the same 3 emeralds, 30 times the carrying, and the shortest wait is the Shepherd at 1.9 real minutes of shelf, gathering not counted.

None of the usual village mechanics touch this merchant. He is not attached to any village, shows up on his own near wherever you are playing, keeps two leashed llamas with him, and leaves again after something like forty minutes to an hour whether or not you bought anything. There is no workstation and no job block behind his stock.

He never buys kelp back. His purchase list runs to six named items and kelp is not among them, so this trade is one direction only, your emeralds for his stock.

Whether the trader in front of you has kelp comes down to the slice of his full list that spawn happened to draw, never the entire set at once. Naming a trader who does carry it stops him from despawning, turning a passing encounter into a shop you can revisit.

A full stack from the trader costs 192 emeralds across 64 trades, which is a lot to pay for something that grows in unlimited quantity on any patch of ocean floor. This trade is worth it mainly when you are landlocked and need fuel or food right now, not as a standing supply line.

Check the rest of what the same trader is carrying before moving on. Also on the wandering trader's list: sea pickle, glowstone, nautilus shell, dye, tropical fish bucket and pufferfish bucket.

Kelp grows upward on its own in any deep enough water, which makes it the easiest renewable resource in the game to farm: plant once, harvest the top, and the stalk regrows. Dried and pressed into blocks it becomes furnace fuel that outlasts coal.

It has a second use builders care about. A column of kelp turns a shaft of water into a source block all the way down, so an elevator or a drop shaft can be filled by planting kelp and then breaking it again. Eating it raw restores very little, so the plant is worth more in the furnace.

Kelp is a source block of water with a plant in it, which means a column of it is swimmable and drops you nowhere. That is the basis of the bubble lift and of every water elevator that does not use a soul sand column.

Placed in a single block it also waterlogs, so kelp is a way to keep water where a bucket would wash it away.

Composted, kelp is one of the better fillers, so the same farm that makes fuel makes bone meal with the part you do not dry.

The wandering trader charges 3 emeralds for a single Kelp, which only makes sense if you live nowhere near an ocean. Planted on the seabed, Kelp grows toward the surface and grows back after you cut it, as long as the bottom piece stays. How to Build a Kelp Farm with Auto Smelter turns that into a steady supply, smelted and ready.