Buying and selling happens by talking to shopkeepers.
Any character with items under Shop Inventory on their profile. The stock belongs to the character, not the building, so it goes wherever they go.
Shops come stocked when they're made by the Location Wizard (for shop-type buildings), by quick options, or by settlement templates. You can add or change stock yourself any time. See Editing a shop.
In a conversation with a shopkeeper, their wares appear in a panel at the side of the screen (on mobile, behind the shop button). It lists each item, its list price, how many are left, and any enchantment.
To buy something, ask for it. The shopkeeper completes the sale, the money leaves your wallet, and the item arrives in your inventory.
Offer the shopkeeper something from your inventory. If you agree on a price, that's what you're paid. If you don't discuss one, you get half the item's value. Whatever you sell joins their stock.
Open the character in Atlas → NPCs and find Shop Inventory. Each item has:
Changing an item's price or type fills in a matching stat. Set the price first, then adjust the stat if you want something different.
Each genre has its own currency: gold, coins, dollars or credits. See Money and economy.