How Does It Work?
There are hundreds of NPC merchants scattered across the world of Atlas, who will buy and sell everything from ores and armors to crops and alcohol! Most of these shops have "dynamic pricing," meaning that the price of an item will fluctuate based on the amount that shop currently has in stock. The higher the stock of an item, the lower the buy and sell price becomes. If the stock amount lowers, the item becomes rarer, and the cost goes up!
Every NPC Town has a handful of merchants that will buy and sell goods. (Currently, the only types of merchants have static, unchanging prices are Siege Engineers, Wand Vendors, and Fishmongers.)
Most other shops have dynamic pricing that will change depending on the merchant's stock. Keep in mind that merchant prices fluctuate independently of each other - if the Potion Merchant in Nauru isn't buying something at a good price, they're probably inundated with that item - try the Potion Merchant in Port Qi'Shen instead! Or simply wait for the Naurulian merchant's stock to return to normal.
Every shop has a Base Amount (generally 100) that is the "standard" amount of stock for an item. Every hour, a merchant's stock will slowly regenerate back towards that Base Amount. The rate at which stock regenerates differs from item to item and shop to shop. This amount is not displayed to players, but generally falls around 10% per hour (depending on the rarity of the item.)