Xjyuk

In the world I'm making, humans trade with a variety of species, one of which is a society of merpeople. Humans typically acquire goods and services in the form of fish, pearls, under sea flora for medicine, navigation assistance, and passage through merfolk waters. What I would like to know is what would a race of merpeople likely want in exchange for goods and services?

Potentially Key Details:

• Humans have the highest form of technology at something between late medieval and mid age of exploration depending on region where the merfolk are early to mid medieval generally speaking.




• Metal work is limited for the merfolk (unless someone can tell me how metal work would function underwater but that may be a question for another time).




• The merfolk live in large underwater cities often built from manipulated coral or other such underwater structures.




• They farm fish by raising generations within underwater cages like modern pisciculture.




• The common trade good in this world is gold coins but, generally speaking, most would still deal in bartering.

Physically the merpeople are in appearance human above the torso with their lower half being like a fish (Disney's little mermaid style) with the inclusion of gills on their neck and more fishlike eyes. They eat about as much as the average person would and can digest the same types of food as a fish would as well.

Merfolk society is not one united kingdom; there are many kingdoms under the sea that have their own established borders and trade routes among each other that may interact with human traders in different ways.

Let me list a few ideas:

1. Metal products. It is very unlikely that there would be many merfolk willing to spend prolonged periods of time outside of water just to craft metal objects. Metal products can be many, from weapons(most likely harpoons, or other weapons that are meant to stab) to tools, jewelry and ornaments. While rare, you don't need to use much of metal for many tools, the main parts of tool might be from something else. For jewelry, metal being rare only increases their value.




2. Ceramics(porcelain, pottery, etc.). For the same reason, it's unlikely that pottery would be mass produced by merfolk. Still it is decent way to store things. This might include some "chemical weapons" sealed in pottery. For example quicklime could be used in underwater warfare. When pot full of quicklime is thrown at someone, breaks and quicklime escapes into water, prepare for some burns.




3. Boats. It might seem counterintuitive, but boats can still likely be of use to Merfolk. They're a very simple way of transporting a very large amount of comodities of any kind. These boats would be however have to be crafted with merfolks in mind, using tamed fish or whales to pull them.




4. Building materials could still be of some use to them. However I am not sure what would be ideal for merfolk civilization.

Edit:

5. Nets! How could I have forgotten about nets. I'm pretty sure nets would be of use for merfolk. They don't need to be used just for fishing, they'd make - decent windows. While merfolk can probably make something similar out of underwater plants, it could still have use, or it could at least be treated as an exotic commodity (land-folk made means exotic).

Many of the ideas I had (metal goods, ceramics, etc) are well-described by other answers. So I'll focus on the ones that are not yet listed.

Storage

Even on land, secure storage of goods is an important service. Banks can secure actual goods (think safe deposit boxes) or virtual ones (a tally of your money, so you don't have to hide it under your mattress). And there are many non-banks that can keep goods safe. Safety here means both safe from others stealing it and safe from the environment.

Accumulating Wealth

Merfolk may also wish to own things that are not stored away but, rather, kept in production. For example, a flock of sheep. Milk, meat, and wool might be of direct use to a merperson, or they could be used to trade for other things or simply to accumulate wealth. Owning a flock and hiring humans to caretake it might work out better economically than simply trading for the actual goods you want each time you want them.

Depending on the laws of the human society, it is possible that a merperson who owned land (even if they could not live on it, or even visit) might have rights that they would not have otherwise. Voting (if it is a society that votes), or the occasional ear of the monarch, or other legal niceties accorded to landowners.

The goods the merfolk produce could pay for these things and their upkeep, and pay the humans they hire to look after them.

Record Keeping

Another poster mentioned writing and I agree it's a strong choice. Recording the history and stories of the merfolk, as well keeping records of their financial transactions would be very useful.

Don't think too hard - just look at real life

In a way you kind of answered your own question. In real life we have lots of countries that buy/sell each other loads of things. In the past, we had barter, but many governments minted their own coin. This goes back to at least Ancient Greek times - the Chinese even had paper currency!

Alternatively, larger deals incorporated other things. For example, in exchange for a huge amount of gold and other treasures, the Pope once sent away Atilla the Hun's army. Plus, you even say that there are other kingdoms - the merpeople are not unified. I'm going to try and answer 2 questions at once: what do the merpeople want from humans, and what do the humans want from merpeople (I know you have the second part covered, but maybe you'll get a new idea). So, a few things that can take place:

Currency: Merpeople pay for human goods with either gold/precious gems they find underwater, or even sand dollars! If they are a hierarchical civilization, I have no doubt that each kingdom has its own mining operation, and not all areas of the ocean are equally as bountiful. They'll also want to buy minerals and rocks from humans on land that are rare underwater for decorative reasons, capital, or other reasons.

Luxury/hi-tech products: Greek fire (fire that doesn't get put out with water) would be found easily in the land of merpeople but its secrets has been lost above ground. Souvenirs from shipwrecks are also possible - you can orchestrate entire deals with this if the ship is valuable enough. Rich merpeople would want to buy stuff that is common on land, but impossible to craft and grow underwater - like human architecture or palm trees. Paintings would be uncommon underwater as the paint would just go everywhere, so paintings would be popular purchases also.

Military support: Hire a mercenary navy. Imagine fighting a force that comes from under water and attacks you, and can't be drowned at sea - they're at home when they're at sea! Tech inferiority or not, this is going to be terrifying. Rich humans will buy mercs from the ocean. The merpeople may also hire mercenaries to kill any humans overfishing in their waters on land itself.

Food: Fish - obviously the cheapest and most consistent product can be sold to humans in return for all kinds of human food, like coconuts, various exotic meats like chicken and lamb, etc.

Trade Goods

What They Have...

• Fish


• Medicinal Underwater Plants


• Psychotropic Underwater Plants


• Transportation (storm-proof undersea barges)


• Coral and Geodes


• Nonferrous Metals


• Shipwreck Cargo

What They Want...

• Rustproof Metal Tools and Weapons


• Bags/Jugs of fresh water


• Buoyant wood - it is like anti-gravity to them

A pair other ideas about what merfolks could desire from surface people:

• air: surface people could easily fill animal bladders or goatskins
  with air (they have lungs) and sell them to the merfolks: they could be used as
  entertainment (think of bubble fountains for the VIP parties) or to help lifting heavy weights underwater. Of course, if merfolk have lungs too, they could fill them on their own*, so, even if goatskins and bladders would still be useful, they would be of less value




• transportation: swimming for long distances would be very tiring, and probably only the richest ones could afford to breed a dolphin or a giant seahorse to ride. But merfolk could easily pay for a lift on human ships: these could feature some kind of net (or even a small undersea cabin) underwater, where the merfolk can enter and wait for the ship to arrive near their destination