I'll preface whatever I'm going to say next with the following:

I love this mod, I want it to be the best.

Here's some issues I discovered and some solutions on how to fix them:
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ISSUE: you can click fast to sell/buy things more than once, thus going over the cap and ending up with negative values. It can cause your game to break, where you suddenly are able to buy out someone else's currency this way while creating a black hole in your national bank, owning like -500mil of your own currency.
SOLUTION1: as soon as the game registers that you clicked buy/sell, the button is locked SOLUTION2: you can create some weird ass table where any buy/sell order is put in it and then run through the entries in the table to verify + execute it every months or two, on some random day, like 15th, so that it would not clash with autosave dates. What do I mean with this? You take away the direct ability of transacting between countries/players and put a 2 step process in it. how would it work? If a player clicks only once to sell 20 mil of british sterlings to USA, that transaction is added to a table that would be calculated at some arbitrary later point in time. the game would go through entries in the table one by one and then resolve any transactions with the logic of (does either party have a valid amount of money/goods) + (have I processed this players/countries sell order) and (does it exceed the limit set by the game on how many units of cash the player/country was allowed to transact)
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ISSUE: Input value checks are not done properly. If a stockpile of raw materials shows that you have 7.5 million units of iron, and you sell 1 mil of it to someone, the amount in stockpile drops by 0.05, leaving you with 7.45 mil instead of 6.5 mil.
SOLUTION: Recheck how the calculations and markings of quantities is displayed for raw materials market.
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ISSUE: the ability to switch your country to a different currency should not exist that easily. SOLUTION: It should be locked behind a boolean check of IS YOUR OWNED DOLLAR QUANTITY SMALL? and WHAT IS THE HIGHEST AMOUNT OF CURRENCY YOU OWN?
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ISSUE: transaction limits are not enforced. Basically you can buy 70Mil sterlings when the total trade limit for that year is 20Mil of sterlings.
SOLUTION: Re-check how input validation is done for this function. Currently it only checks your owned stockpile of cash in the bank and not how much you are inputting.
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ISSUE: Players and AI are bypassing the bond trade restriction. By default any country starts with bond purchase setting as off, but that does not stop countries and players from buying bonds from players/AI, effectively either becoming rich af or crashing their economies. SOLUTION: Recheck how the validation of this function works. I've seen AI and players buy more bonds than there are buttons. It's weird actually. Maybe implement a table for this function too where the game would go through added transaction inputs and would make checks for valid transaction inputs and discard invalid ones
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ISSUE:Currently the way variables have been set up allows for negative values, which is a bad choice as many calculations are not working properly with them. Examples: play as india -> switch to Ruppee as your currency (which should not be easy in my opinion in the first place) -> set the buy on british sterling to 70 mil ( which will = 140 mil rupees, an amount you have in your central bank at the beginning + the game does not do a validation on the amount of currency you are planning on interacting with with any coutry. India's limit on transaction with GB is 20 mil, so you should not be able to transact 70 mil sterlings in the first place) -> click fast to buy more than 70 mil sterlings (depending on how fast you click, you might be able to buy up to 350 mil sterlings or more!) -> the result is that sterling is inflated, you own now a negative value of 800mil rupees and Great Britain owns either no sterlings or a negative value of sterlings.
SOLUTION: Enforce positive variables.
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ISSUE: Moving your capital breaks the game and removes all sorts of items you owned in your central bank and breaks the game in weird ways
SOLUTION: disable that button. Just add some sort of requirement that is impossible to meet and make a tooltip for that requirement display "Not allowed due to game-breaking bugs".
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NOTE: If you are going to store and discard transaction inputs in a table as per solutions above , you have to make sure that this is done properly to avoid memory leaks or junk data being stored somewhere. Make sure that you set up a junk collector (or whatever it's called) to clean up data without leaving anything behind, otherwise the game will become unstable.