V1.10.0: NPCs change money and sell things

- An NPC can change gold and cash at a rate of their own ("2 gold = $15"),
  the same both ways. Only gold and cash are taken: no silver, no copper
  and no change, and the amount has to be a whole multiple of the rate.
- An NPC can sell weapons and items from the campaign pools, each with a
  price in any coin or in cash, a level and an optional stock. The wares
  follow the NPC into every quest. An encounter can sell things itself
  too, for a stall with nobody behind it.
- Trading is open only in an NPC quest in progress, never in a battle,
  with an NPC who is in the scene and standing, for a character who is in
  it. Players see every rate and ware and trade for their own character
  in a new Trade card; the DM can trade for anyone.
- Only the DM sets trade up: "Trade" on an NPC's row on the campaign page,
  and the DM's part of the Trade card for an encounter's own wares. A
  ware's price and stock can be edited afterwards.
- Every exchange and purchase is written to the character's money log.
- All packages at version 1.10.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ITEMS.md

@@ -15,7 +15,7 @@ level up the same way weapons do), [SPELLS.md](SPELLS.md),
 4. [Item levels](#4-item-levels)
 5. [Dice on items](#5-dice-on-items)
 6. [Starting items](#6-starting-items)
-7. [Items as rewards](#7-items-as-rewards)
+7. [Items as rewards and for sale](#7-items-as-rewards-and-for-sale)
 8. [Permissions](#8-permissions)
 9. [What is not modelled](#9-what-is-not-modelled)
 
@@ -121,13 +121,20 @@ In the builder's equipment step a new character can pick items that are:
 Starting items and starting weapons share the campaign's "starting items"
 allowance. Each arrives at level 1, quantity 1, unequipped.
 
-## 7. Items as rewards
+## 7. Items as rewards and for sale
 
 On an encounter the DM can line up an item as a reward, at a chosen level and
 quantity, and later award it to a character. Awarding adds it to the
 character's inventory, stacking with any they already hold. See
 [ENCOUNTERS.md](ENCOUNTERS.md).
 
+The DM can also put an item **up for sale**, at a level, a price (in gold,
+silver, copper or cash) and an optional stock. An NPC can sell it, wherever
+that NPC is met, or an encounter can sell it itself. Players see what is for
+sale and buy it in an NPC quest that is in progress: the price leaves their
+purse and the item joins their inventory. See [NPCS.md](NPCS.md), section 5,
+and [ENCOUNTERS.md](ENCOUNTERS.md), section 10.
+
 ## 8. Permissions
 
 | Action | DM | Owning player | Other players |
@@ -136,6 +143,8 @@ character's inventory, stacking with any they already hold. See
 | Add, change or remove pool items | Yes | No | No |
 | Give an item from the pool, or take one away | Yes | No | No |
 | Give a held item to another character | Yes | Yes | No |
+| Put an item up for sale | Yes | No | No |
+| Buy an item that is for sale, in a quest in progress | Yes, for any character in it | Yes, if the character is in it | No |
 | Change quantity or level | Yes | No | No |
 | Equip or unequip | Yes | Yes, if the character can use it | No |
 | See a character's inventory | Yes | Yes | Yes |
@@ -146,6 +155,7 @@ character's inventory, stacking with any they already hold. See
 - Armor and shields as items
 - Using up items automatically, charges, and attunement
 - Weight, carrying capacity and containers
-- Prices, buying and selling (record the gold change in the money log)
+- A price on the item itself: a price is set each time it is put up for sale
+- Selling items back (record the gold change in the money log)
 - Swaps as a single step (each side gives separately), and asking the
   recipient to accept