I take advantage of that all the time. I sell any saltpeter I mine to the npc. Also I constantly find people selling it for less than the npc will buy it for, so I buy them out and make a quick penny.This is a perfect example of the type of inflation that the game's economy has undergone.
Saltpeter's price floats using a "commodity trader" algorithm. As players purchase it from the NPC, the NPC raises its price, simulating scarcity. As players sell back to the NPC, it lowers its price, simulating glut. Thus, the price of the item is determined by actual supply and demand.
The alternative to buying saltpeter is to mine it. It can be mined at a rate of 1,000 units or more per hour in a dungeon, with a mediocre luck suit and GM mining skill. It can also be mined at a lower rate from the safety of the deck of a ship. This means it is definitely possible to make gunpowder by investing time instead of gold into it.
The NPC pays 75% of its asking price when buying the commodity item from players. So if they're charging 500gp per unit to sell, they will pay 400gp per unit to buy. So, if you can mine 1,000 saltpeter per hour, that means you can sell it to the NPC and make 400K gold per hour. If you're selling saltpeter, you also have the option of selling directly to other players, undercutting the NPC price, to make more than what the NPC offers.
I also do this for arrows, bolts, and iron ingots. Many of these items you can find selling on player vendors for less than the NPC will pay you. All because people would just buy from the npc for convenience and let the prices go up and up.
I really think many more things should follow this pricing scheme and this could be a great gold sink.
Items this could currently be applied to.
Clothes - those that still do tailor bods just buy cheap clothes to fill their normal bods and turn the bod in hoping for a better bod. Problem is the clothes prices don't change. So for years the tailor npcs have been getting the short end of the stick. Now if clothes were in commodity pricing scheme, the price might rise enough where it would be economical for me to make the item rather than buy it from an npc. As a curious side effect, players would be able to loot these items that are rising in price and sell to a tailor for increased revenue. This could greatly increase the gold potential from lower end monsters.
Leather Armor (including studded and female) - ditto from above.
Metal Armor and Weapons - ditto from above.
I think the crafter npcs should realize noone (or at least very few) actually buy their wares for use and thus get on the bandwagon and make some money for themselves (aka goldsink)
Tools - Many don't spend the time to make tools or buy tools from another player. Sure they have more charges and such, but no one wants the hassle (of making and stocking or selling on a vendor or finding a vendor who keeps stocked etc.) when you can just recall to the npc shop and buy them fairly quickly. If all tools went to commodity pricing perhaps it would become more economical for me to buy a batch of say mortar and pestle from a player tinker rather than just recalling to the alchemist shop and buying 20 of them for 160 gold. Even if I don't change my practices, at least more gold is leaving the economy. And perhaps people buying tools would visit some newer cities looking for cheaper tools.
Reagents - I still haven't made up my mind about this especially with faction vendors and such. But I could still imagine a scenario where reagents were on commodity pricing. The price might rise soo much that it would make it profitable for me to stop and pick up the spawning reagent of the ground lol.
Now if you add in some new items that currently aren't traded on the npc market, then you can have some real interesting things happen with the economy. A vendor that buys colored ingots, orange petals, greater heal potions etc. I believe looking through the spoilers for the latest publish they are looking to implement this but keep it in the player realm. I think this might be a mistake, and I believe this should have been kept in the npc realm. I believe it is the greed of the players and there concepts of what they should get for an item that really screws over the economy. For example people selling blackrock stew on my shard are now selling it for 65k to 100k per shot roughly. I personally sell it for 5k a shot right now on a couple vendors. I think my price is still high and think it should settle in between 400 to 1000 gold. I've been on a mission to get the price lowered on this shard wide and in less than a month have accomplished this somewhat. It used to sell for 250k to 1mill gold. If a npc would buy and sell blackrock stew using the commodity pricing scheme, we could find out what the real value is as the price would be market driven.