Q:

How much money can I make up with class features?

So, recently I've started RPing a Pathfinder Gunslinger in a city (we're running Curse of the Blood Throne with early firearms). According to the Gunsmithing feat, I can produce 1.000GP worth of ammunition daily, for 10% raw materials cost. The cost of a keg of black powder (5lbs) is 1.000GP. 
Can I sell this black powder, effectively netting myself 900GP/day of work?
If I can, could a wizard sell scrolls in a similar way? If I can't, is there a mechanical reason or it's more a GM/Campaing thing? How does this compare to the use of Profession and Craft skills (from a money-making point of view)?

A:

Generally speaking, Pathfinder has extremely minimal economic rules. They aren’t a focus of the game and they don’t stand up to even the barest critical inspection. There are a variety of ways to use class features to make extreme wealth.
But you usually shouldn’t; wealth in Pathfinder is also heavily tied to power, which means it is heavily tied to character level. The game is designed expecting that characters of a given level will have roughly a given amount of wealth—no more and no less (or, more accurately, not substantially more or substantially less). The game expects the GM to manage this “somehow” and isn’t super-explicit about how—we have a question about that from the GM’s perspective.
If you look at my answer to the linked question, I spend some time talking about getting “buy in” from the players. Pathfinder is designed for a certain kind of game: PCs are specifically adventurers, and they’re expected to go on adventures. This is the primary way in which wealth and level are kept in sync: wealth generally comes from adventuring, which also gains you XP so you level up as you get wealthier.
So for you, as a player, you should generally respect that expectation and create a character who will go along with it. Attempting to play a character heavily interested in and involved in the economy of the world simply does not work well—Pathfinder doesn’t offer rules to handle it, and that sort of character, that sort of game, ignores the vast majority of rules Pathfinder does offer, which mostly pertain to adventuring. Save the mercantile capitalist character for another game in another system that actually supports that kind of thing, because Pathfinder doesn’t.
Which isn’t to say a Pathfinder character never engages with the economy at all; that certainly isn’t the case. It’s just not a focus, can’t be a focus, not even remotely. So your gunslinger certainly could make some money on the side this way. Your GM would have to figure out how to handle that, how to handle you having more wealth than expected for your level, and so on. You should discuss that with your GM before doing it; it may be a headache he doesn’t want or need. At the same time, though, he might be happy to do so—the gunslinger class, unfortunately, is hideously weak and needs all the help it can get.

A:

To quote the core rulebook:

Selling Treasure
In general, a character can sell something for half its listed price, including weapons, armor, gear, and magic items. This also includes character-created items.

"In general" means that selling for half the listed price is not always possible. But that players can expect to easily sell whatever they want for half the price without going through the trouble to figuring out if and who wants to buy their items.

Can I sell this black powder, effectively netting myself 900GP/day of work?

No, you could if you had enough daily demand for 5 lbs of black powder. Which is very unlikely, unless your country is at war and firearms are common (probably not the case for Curse of the Crimson Throne). However, that decision is up to your GM.
Pathfinder does not simulate a mechant lifestyle the way you think. It does that by using the Profession and Craft skills, and the downtime system, an optional system designed to expand on those skills from Ultimate Campaign. That said, you will notice that depending on how good of a seller you are, you might make around 1-2 gp a day without difficulty, but anything more than that means there is some kind of event that increased the demand for your products.
So, even if you have 10 kegs of gunpowder on your storage, you might have to wait for weeks before another gunslinger (or someone really interested in gunpowder) walks into your shop and buys one of your kegs.
What this means is that, even if a blacksmith can craft one longsword per day, the settlement he lives in does not have demand for that many longswords. So his wares will be sitting in his storage until a buyer shows up and they agree on a fair price for the goods.
In other words, even if you make 5 lbs of gunpowder a day, either the market has no demand for all that gunpowder, or you are simply a terrible seller.
Rules as written, yes

If you have access to a gunsmith’s kit, you can create and restore firearms, craft bullets, and mix black powder for all types of firearms.

The Gunsmithing feat does not allow you to craft a keg of black powder, it allows you to mix black powder, which by the definition given on Ultimate Combat, is a different item altogether. However, a keg of black powder is about 100 doses of gunpowder, and you can craft doses at 10% (1 gp) of their market value (10 gp). Since you are limited to 1,000 gp, this means you can craft enough black powder on a single day to completely fill a keg of black powder.
So, if you craft 100 doses (worth 1,000 gp), you have to pay 100 gp in raw materials and you could sell each dose back at the market for 500 gp, for a profit of 400 gp.
Of course, assuming the GM is okay with that.

