By having costs lower then what they make selling their wares, same way any other enterprise makes a profit. No merchant is going to spend the same amount buying a good that they are selling the good for. There's no profit in doing that.
In the case of salt, it's just as liquid between the seller and buyer as it is between that buyer as a seller and the next buyer. Silver doesn't lose any value between one transaction and the next, and salt likewise is just as liquid of a currency from the previous sale as it is for the next purchase.
It might not be government-issued, but salt is still just as valuable as silver.
The D&D economy doesn't devalue something just because there's a lot of it; otherwise,
spending dozens of thousands of gold pieces on magic items wouldn't work. Flooding an economy with that much money would mean that gold would simply cease to have value, which would wreck the whole game. Salt is identical in function (except you can eat it, if you want).
Let's say the salt mine produces 20 pounds of salt an hour, and miners work for 8 hours a day five days a week. The mine has 12 miners working for it, six per shift and two shifts. That's 16 hours of production a day, or 320 pounds of salt produced a day. Let's assume the mine sells the salt for 5 copper per pound. That means in one week the owner makes 1120 silver, or 120 gold from sales. The miners get paid one silver per day each. This means that in one week 60 silver, or 6 gold is spent paying the miners. Also subtract costs to transport and costs for materials used to brace the mine. Oh, and replacing picks when needed. This tells you the profit the mine is making. If the miners are making 3 silver a day (which is more reasonable, probably) that's still only 25 gold spent on wages for the miners.
Salt sells for 5 gp per pound, just like silver, so none of that is accurate.
No, it's a trade good. There's a bit of a difference. A currency is minted and backed by the ruling government. Trade goods are goods that have accepted value. Your cattle might have an estimated value of 10 gold a head. But unless you can convince the person you're trading with, you're not going to get that much value from them. And you're not going to be able to walk up to a merchant and give them three calves for 30 gold worth of products or services without some serious haggling. Especially if the merchant doesn't normally deal in livestock.
Sure, it's a trade good, just like bars of silver. That means that it can be spent just like money, because it
is money. It just happens to not be minted coins.
Currency is anything that can be used in trade for something else, and trade goods in D&D are usable as currency. And deflation and inflation don't exist, else the magic item trade also couldn't exist.
This says nothing about merchants being required to accept trade goods in leu of currency. In fact, you try paying a blacksmith with salt or bags of rice, the blacksmith is likely to ask you what he (or she) is suppose to do with that and then demand you pay in gold.
"You use it to buy something else you want. It's worth X gold pieces, after all."
Remember these aren't modern day economies.
But even now, you can't typically spend "a handful of rubies" at your local Walmart and expect to get anywhere. However, if I were to offer you that handful of rubies in exchange for something you have that I want, are you generally going to say no*?
*Barring extreme sentimental value or other unusual situations, of course.
This again doesn't say every merchant and tradesman must accept a trade good as if it was currency for the full listed market value in the book, just that it's easy to find a buyer willing to pay the local market value. Which may not be the same as what you will pay if you buy the trade good from said merchant. Trade goods are things you might be expected to buy cheaply in one area, then sell for more in another area that has a shortage of that product.
Rules source on this?
Because as far as I can tell, "5 lbs of salt is worth 5 gold pieces." That's it.
Unless it turns out that you are the BBEG, and the adventurers have been hired to take you out because you're destroying the local economy. In which case, you watch in horror as that group of rag tag but surprisingly competent adventurers tear through all your defenses like they weren't there before coming after you.
There are ways to hit WAY above your weight class as a prepared wizard-type. Especially with all the "silver" you can (literally) eat. They're known as "Batman wizards" for a reason. I mean, there are ways to take out epic level monsters at level 1, if you know what you're doing. As an example, take Precocious Apprentice
(Ray of Stupidity). Anything with a decently low ranged touch AC and an Int of 1 or 2 (
which do exist, even in epic) is basically an auto-win. Of course, that's assuming you don't roll a nat 1.
