So, if tariffs and your stockpile are what regulates how efficient your factories are, how do you see and ensure that they get all the inputs they need? My factories are all bloody useless and state-subsidized.
I'm kinda new to the game though.
Erm, well efficiency refers to the ratio of output to input. So if efficiency is higher then you'll make more goods for the same inputs.
I have never fiddled with national stockpile to any avail. I've tried buying up timber and releasing it to capitalists but they never took it. I just ignore stockpile and never have any problems. If you can buy the item your POPs/capitalists need, they've got access to it as well, so they hardly need you to serve as a middleman unless supply is intermittent. At least that's been my experience.
Basically the game works on:
POPs buy first, then nation, then factories/capitalists (AFAIK, but definitely POPs first).
If demand can't be met on your internal market (your country, plus sphere of influence) they look on the world market.
On the world market the country with the highest prestige buys first, so it works its way down. It works the same within a sphere of influence.
If the internal market doesn't buy all the day's production of an item it goes on the world market. If it isn't all bought there it is "dumped in the sea"/disappears unsold.
So to ensure your factories get inputs, well ideally it's within your country. If it isn't, try to put it in your sphere, eg if you need dye try sphering a country that produces dye. If it's not in your country or your sphere then you buy it off the world market, assuming there's enough supply to fill your factories' demand by the time your turn comes. If it isn't available on the world market you've no easy options. You'll need more prestige to move up the food chain, or else have to conquer territory with the item, or else pry off a sphereling that has the item from someone else's sphere of influence. If you can't do any of that then build new factories with less exotic inputs!
Certain factories are good money spinners: cement is always good. Glass is good usually. Steel's often a tidy earner. Others are awful: most people revile canneries for their money losing abilities. Military goods often don't turn profits. Steamers and clippers are borderline.
Tech affects this. I've said it somewhere, I think it was the economy evaluation thread. Basically the most advanced nations will be flooding the market with way more supply than there is demand through tech benefits + volume. Prices plummet. If you're behind on tech then you'll have a hard time turning a profit even if you can sell your output because the flooding by the advanced nations has driven the price low (and yet arguably, not low enough).