Ok, after testing this out for a bit here is a short overview of how it works and my thoughts:

Contrary what many seem to believe, this mod does not add any crazy complexity to the game (unlike say MEIOU and Taxes (which I have played extensively)). It's pretty much fire and forget, you don't have to engage with it at all.

What it does is it takes a good that is produced in a province and based on dev/goods produced it generates "units" of this good in the mod (you can check all the relevant stuff in various decision tooltips, it's sorta self-explanatory).

So say you start as OPM Ragusa. Ragusa produces salt. So at game start, this is the only good you produce. The mod will automatically accrue a stockpile but also sell off the good to the open market. You also have demand on other goods than salt though, in fact you will have demand on most goods except some new world exotics right from the start. The mod automatically buys those from the open market, but does NOT accrue a stockpile, which makes this a bit confusing at the beginning. However, if you hover over the Stock Income tolltip in decisions you can see how much you earn from selling salt, and how much you spend from buying everything else.

In addition, through tech and partially based on terrain type, you can also build buildings that produce a good. Salt Mines seem to be creatable in every province, but these buildings always produce a fixed amount, like 50 units. In comparision, 45 dev ragusa with the salt tradegood produces like over 300 (can't check right now, but it's A LOT).

So your population (I guess based on dev) creates demand and then there are other buildings that also create demand, eg. a brewery will create 50 demand for grain and give you 1 yearly tax but also +5% tax for the province.

Based on global supply and demand, prices rise and fall in addition to some rando events that modify prices.

You can also set consumption taxes on every good. This does not actually generate any income, instead it reduces dev-based demand by a fixed amount. So if something becomes too expensive to import, you can tax it to decrease demand and save money. This does increase unrest though.

If you want to stockpile a good without producing it you have to manually buy it, however once you have a stockpile, demand will be satisfied from that rather than importing. You can however block your stockpiles from being used, effectively making an untouchable stockpile. You can also seperatly disable import of any good, allthough I have no idea why you ever would want to do that, as having no stockpile and having disabled import of a good at the same time will give you a "severe shortage of good" modifier which is absolutely brutal. These are so harsh, they can and probably will game over you if left unadressed.

You can alternatively set a maximum budget to use for buying a certain good each month (default is unlimited for every good).

So now that we have established how everything works, here are the problems with this mod:

1) Balancing is totally whack, you earn much much more from selling the stuff you have than you need for buying remaining demand from dev. This causes not only you, but literally everyone to have obscene amounts of money. You as the player will earn more though because the AI doesn't utilize the buildings properly (they do build some but they could build much more). It basically turns the game into mega easy mode.

2) The system is disconnected from normal production and budget. While some vanilla production buildings (manufactories) are somewhat changed, vanilla production mechanics are as always and the entire stock thing goes on top of that, so you earn like double for your production.

Also, and much more critically, the money you make through the stocks is I guess event/on_action awarded and does not show up at all in the budget window. You can check via Income tooltip in decisions how much each good makes/costs you, but that tooltip lacks a total sum. This total sum of what you sell minus what you have to import is awarded on the month tick seperatley from the normal vanilla money tick.

This causes the AI to get mega ♥♥♥♥♥♥ because since they don't "know" that they make a crazy amount of money, they can't utilize it. IE, in my Ragusa game I went 20 units above force limit and game said I was losing 12 ducats a month but in reality I was still making 5 ducats a month net.

So by 1480 I had like 5k monies with 3 provinces. Ottos had like 20k saved up but wasn't using it up properly because their vanilla income was only "okayish".

Interestingly, if income from the system were to be counted properly in the budget, I think the game would suddenly become incredibly difficult as the AI would go absolutely bonkers on units and aggression. This would probably be a lot more fun but also crazy.

3) From what I can tell from my testing, the absolutely brutal "severe shortage of good" modifier will only ever appear if you go out of your way to make it appear ie prohibit import and make sure your stockpile is empty. So the mechanic seems basically pointless. I guess the idea is that all the demand you have should strongly counterweight what you make from selling the stuff you do have, in a way where the import costs should be able to be greater than your export revenue and maybe I just got insanely lucky with prices or something, but that scenario just never seems to happen at all, like not even close by several orders of magnitude.

Overall really interesting concept that has the potential to spice up things a lot without becoming absurdly complex like MEIOU, but as it is right now, it's basically just "very easy" with extra steps.