Found a new one (at least I have not seen it mentioned anywhere), and it probably doesn't need to be mentioned since it is obviously more a cheat then an exploit and involves a mod, but I will do it anyway:
The mod in question is
Quick Deals, which is (supposed to be) just a UI mod and I believe pretty common. It is about selling strategic resources.
When you use the mod you can just put in the kind and amount of resources you want to sell. You can do so by clicking on the icon on the left repeatedly, or click on the symbol in the middle and put the amount in via keyboard that you want to sell. If you put a number bigger then your available resources, it automatically gets reduced to your stockpile.
After I sold to Nubia, I have 44 Iron left (50 -20 I sold, +14 I got this round, this how the game works and still correct. The screenshot below is what you automatically get after clicking accept deal in the screenshot above.
You can trade the rest of the iron, and increase the amount again by clicking on the symbol on the left, or via clicking on the symbol in the middle and writing the number down you want to trade. And here is the kicker. If you use the method of clicking in the middle and putting your number in, the maximum you can trade is not what you actually have left, but the amount you had before you made your first trade. In this instance 50.
Again, selling 40 to Gaul. The next "suggested" deal shows "10" iron in the middle (50 now - the sold 40), even though we have only 4 left, and by clicking in the middle and putting anything in we want, we get limited to our original limit of 50, even though we only have 4 instances of iron left. (as can be seen on the left side).
If we had more AI interested in buying, we could repeat this process several times more, even though we would have no Iron left.
This works with all strategic resources. But only if you have more in stock than you want to sell, because it only works if the "suggested" deal in the middle is still available after we accepted a deal. If I would close the deal in the middle in the above screenshot, click on the iron icon to the left (and open a new deal proposal) the new maximum amount would now be our actual supply of 4. Also meaning if we only had 40 Iron and an AI would want to buy all, there would be no "suggested" deal in the middle left and the process would stop.
Since (I believe) AI never wants to buy more then 40 resources at a time, you can always do this once you have 41 instances of one resource (there will always be a "suggested" deal of at least "1", which you can then increase again). I believe I sold 200+ niter at one time in my last game, even though my stockpile was just 50 at that time.