Here is the text for crafting ammunition with the Gunsmithing feat (which Gunslingers receive for free).
Crafting Ammunition: You can craft bullets, pellets, and black powder for a cost in raw materials equal to 10% of the price. If you have at least 1 rank in Craft (alchemy), you can craft alchemical cartridges for a cost in raw materials equal to half the price of the cartridge. At your GM’s discretion, you can craft metal cartridges for a cost in raw materials equal to half the cost of the cartridge. Crafting bullets, black powder, or cartridges takes 1 day of work for every 1,000 gp of ammunition (minimum 1 day).
I've highlighted the important parts.
What this feat is saying, is that a character can craft 1000gp worth of black powder in one day, at a cost of 100gp - and this black powder can be sold at half its value for 500gp.
Unless there is some FAQ or obscure rule I am unfamiliar with, than 1 days worth of work can net a character 400gp per day.
Note that with normal crafting, you need to make a craft check (which is weekly work measured in SP not GP) which generally means that you're doing less than 5gp worth of work in a day by the rules under the Craft Skill.
Most fantasy settings assume that if gunpowder weapons exist, they're so rare that they have no real market demand. This feat says you can make the stuff cheap, not that there's a market for selling it.
Pathfinder assumes "Emerging Guns" if PCs can have access to guns, which is more common than "Very Rare Guns." Aside from which, the CRB says you can sell something (not a trade good) at half price.
And, you're telling me there isn't a market for cheap explosives?
Look, if the GM wants to use GM fiat to keep things sane and say "you can't sell gunpowder" to prevent shenanigans, I'm all for it, but that isn't RAW.
Hell, forget selling it, how many d6s does a keg of gunpowder net you?
The whole purpose of gunsmithing is so that the gunslinger has ammo to adventure with, without losing all the money purchasing it. It's not meant to turn the gunslinger into a merchant.
The idea of gunsmithing is that it allows gunslingers to arm themselves without relying on others since there is normally no place to buy ammo in the default setting. The price of ammo is due to the fact that, even when there are places to buy it, the prices are highly inflated since almost no one buys it.
Even if you tried to sell gunpowder, almost no one would buy it. It is highly dangerous, and usually only useful with weapons that no one has. And even if you did sell guns too, regular bows tend to end up being better, due to iteratives and feats that improve fire rate- for non-gunslingers, only pistols could be used with a lot of effort....and they are objectively worse compared to bows outside of 'they are right next to you distance'. Overall, it is a hard sell, even if they are powerful.
And trying to sell the gunpowder as an explosive is also problematic. While it is somewhat comparable to a fireball and requires less training to blow things up compared to learning wizardry or using a scroll, they are just plain more expensive than hiring a wizard to blow things up for you.
Overall, you need a much larger market for firearms and such before you could make that kind of money...and that market is in a different gun setting where gun stuff costs a lot less. I would only allow selling gunpowder at normal prices in the default setting after the use of diplomacy and on a situational basis (ie- oh, you have a wall that you need to blow up? Well luckily, I have a bag full of holding filled to the brim with gunpowder)
Look, if the GM wants to use GM fiat to keep things sane and say "you can't sell gunpowder" to prevent shenanigans, I'm all for it, but that isn't RAW.
There is no formal RAW economy. There is a brief section, in the book for GM suggestions, on a shorthand set of rules for adventurers buying and selling equipment. There isn't enough RAW to scratch the surface of a real economy, and the RAI obviously wasn't meant to be used this way.
Assuming the content of the GMG applies to manufacturing and economics is also GM fiat, and if you allow it to break your game, foolish GM fiat.
Look, if the GM wants to use GM fiat to keep things sane and say "you can't sell gunpowder" to prevent shenanigans, I'm all for it, but that isn't RAW.There is no formal RAW economy. There is a brief section, in the book for GM suggestions, on a shorthand set of rules for adventurers buying and selling equipment. There isn't enough RAW to scratch the surface of a real economy, and the RAI obviously wasn't meant to be used this way.
Assuming the content of the GMG applies to manufacturing and economics is also GM fiat, and if you allow it to break your game, foolish GM fiat.
Well it might not be 'formal' but there is a working economic model involved.
Interestingly enough everyone is complaining about how economics work on the most basic and fundamental level. First off you aren't 'creating something from nothing' when it comes to wealth -- you are taking base 'raw' materials and adding labor then charging whatever premium you can get away with (both with the public and yourself). This is the premise for the advancement of civilization since barter started.
Additionally this is hardly the first time we have been over this topic so a quick run down of counter arguments:
Wizard's do it better
So what?
And congratulations you are now playing farmers and bankers.
Honestly if this is the part that 'ruins' pathfinder and the gunslinger/firearms for you... well...
Uh, in terms of Golarion, a gunpowder salesman would run into simple issues. All of his potential buyers already know how to make the stuff for just as cheap. Or he garners attention from people who want to keep a lockdown on guns and powder, like the city-state Alkenstar. Or maybe 'they' don't want to buy from some insane person who makes tiny explosions in tubes in their hands.
PFS excluded, "Emerging Guns" does not mean that Golarion is silly with gunslingers or people who want to use the powder.
Actually, there is one place someone would definitely sell gunpowder. Sell it to goblins. Say it explodes and watch the eyes grow bigger. And then hope you have the Run feat and get out of the place before it goes up in flames after you've made the deal.
also, for anything beyond emerging guns, the costs on everything are reduced rather significantly (thus reducing the usefulness of gunsmithing).
but yeah, it's there so that the gunslinger class isn't cost-prohibitive (especially in comparison to the archer, who can just buy a few vials of weapon blanch for his durable arrows and never have to buy/make more again.)