NPC vendors have a limited range of items and set prices that are usually lower than the actual market price of the item to stop inflation. Since NPC vendors only sell a limited range of items, most of the best items and equipment are obtained through monster drops and chests on the higher floors. Unlike player vendors, NPC vendors never need to leave their store to restock on supplies and have specific behaviour, like periodically cleaning the outside of their shop.
Human vendors are players who specialise in trading items. Like craftsmen, merchants have to fill half of their skill slots with battle unrelated skills, but still had to take up some combat skills to be able to battle for goods. Players became vendors to make it easier for other players to sell their unneeded items at a price higher than the price at NPC stores, while at the same time, helping avoid conflict between players after completing a trade. Unlike NPC vendors, the price of items sold and bought by human vendors varies greatly and depends on how well the player bargains with the vendor. Human vendors can have a potentially infinite item range, however, they must acquire all of their stock manually.
Although trading can be done without even having a store, beginner human vendors usually buy a Vendor's Carpet, as it prevents the vendor's wares from being stolen and the durability of their wares decreasing. Merchants who save up enough money may buy a shop that doubles as a home, while merchants who are also craftsmen purchase shops with facilities that can be used to help with crafting.