diff --git a/src/shop.lua b/src/shop.lua
new file mode 100644
index 0000000..0494f1e
--- /dev/null
+++ b/src/shop.lua
@@ -0,0 +1,115 @@
+-- Gold economy: a price catalog for tradeable goods plus the buy/sell math for
+-- a shopkeeper's wares. Wares seeded on the floor by the builder carry a
+-- `price` and `shopId` tag on the ground entry (not the item), so a purchased
+-- item is indistinguishable from a found one once it reaches the inventory.
+
+local Items = require("items")
+local Affixes = require("affixes")
+local Factions = require("factions")
+local Mercenaries = require("mercenaries")
+
+local Shop = {}
+
+local POTION_PRICE = {
+  heal = 15, poison = 6, strength = 45, haste = 25, clarity = 20, cure = 30,
+  confuse = 8, blind = 8, levitate = 20,
+}
+local SCROLL_PRICE = { teleport = 25, identify = 18, mapping = 20, fireball = 35 }
+local FLAT_PRICE = { [Items.FOOD] = 5, [Items.TOME] = 55, [Items.PICK] = 40 }
+local AMMO_UNIT_PRICE = 2
+
+-- Catalog price for any tradeable item. Gear scales with its effective power
+-- or bonuses so a heavily-affixed drop costs more than a plain one.
+function Shop.priceOf(item)
+  if item.kind == Items.POTION then
+    return POTION_PRICE[item.effect] or 10
+  elseif item.kind == Items.SCROLL then
+    return SCROLL_PRICE[item.effect] or 10
+  elseif item.kind == Items.WEAPON or item.kind == Items.ARMOR then
+    return 20 + item.power * 15
+  elseif item.kind == Items.BOW then
+    return 25 + item.power * 12
+  elseif item.kind == Items.RING then
+    return 30 + (item.attack + item.defense) * 20
+  elseif item.kind == Items.AMMO then
+    return AMMO_UNIT_PRICE * (item.count or 1)
+  elseif item.kind == Items.CONTRACT then
+    return Mercenaries.byKey[item.hireling].hireCost
+  else
+    return FLAT_PRICE[item.kind] or 10
+  end
+end
+
+function Shop.sellValue(item)
+  return math.max(1, Shop.priceOf(item) // 2)
+end
+
+-- The price a shopkeeper actually charges once the player's standing with
+-- the merchant guild (see factions.lua) has had its say: sour relations mark
+-- goods up, warm ones discount them. `ground.price` (stamped once at
+-- generation - see builder.lua) stays the neutral catalog price throughout,
+-- so it keeps meaning "what a stranger pays" even as this moves the amount
+-- actually charged.
+function Shop.effectivePrice(game, ground)
+  return math.max(1, math.floor(ground.price * Factions.priceMultiplier(game.player, Factions.MERCHANT)))
+end
+
+-- Buys a warded ground item if the player can afford its reputation-adjusted
+-- price: deducts gold and transfers the item into the inventory. Returns the
+-- gold actually spent, or nil without side effects when the player is short.
+function Shop.buy(game, ground)
+  if not ground.price then return nil end
+  local price = Shop.effectivePrice(game, ground)
+  if game.player.gold < price then return nil end
+  game.player.gold = game.player.gold - price
+  game.player.inventory[#game.player.inventory + 1] = ground.item
+  return price
+end
+
+-- Sells an inventory item at half its catalog price (further scaled by
+-- merchant standing) and removes it.
+function Shop.sell(game, index)
+  local item = game.player.inventory[index]
+  if not item then return nil end
+  local value = math.max(1, math.floor(Shop.sellValue(item) / Factions.priceMultiplier(game.player, Factions.MERCHANT)))
+  game.player.gold = game.player.gold + value
+  table.remove(game.player.inventory, index)
+  return value
+end
+
+local weaponBases = { { name = "rapier", power = 3 }, { name = "war hammer", power = 5 } }
+local armorBases = { { name = "banded mail", power = 2 } }
+
+-- Rolls a small selection of wares for a shop room. Weapons and armour may
+-- still carry an affix, so the shop is a genuine alternative to a lucky drop
+-- rather than a strictly worse copy of one.
+function Shop.stock(rng)
+  local wares = {}
+  local slots = rng:range(3, 5)
+  for _ = 1, slots do
+    local roll = rng:range(1, 100)
+    local item
+    if roll <= 30 then
+      local effects = { "heal", "strength", "haste", "clarity", "cure" }
+      item = Items.potionOf(effects[rng:range(1, #effects)])
+    elseif roll <= 45 then
+      local effects = { "identify", "mapping", "fireball" }
+      item = Items.scroll(effects[rng:range(1, #effects)])
+    elseif roll <= 62 then
+      local base = weaponBases[rng:range(1, #weaponBases)]
+      item = Affixes.decorate(Items.weapon(base.name, base.power), Affixes.rollWeapon(rng))
+    elseif roll <= 75 then
+      local base = armorBases[rng:range(1, #armorBases)]
+      item = Affixes.decorate(Items.armor(base.name, base.power), Affixes.rollArmor(rng))
+    elseif roll <= 85 then
+      item = Items.ring("ring of the merchant", rng:range(0, 2), rng:range(0, 2))
+    else
+      local key = Mercenaries.order[rng:range(1, #Mercenaries.order)]
+      item = Items.contract(key, "contract: hire the " .. Mercenaries.byKey[key].name)
+    end
+    wares[#wares + 1] = item
+  end
+  return wares
+end
+
+return Shop