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/CHARACTERS.md

@@ -585,8 +585,18 @@ behind it.
 - **Cash ($)** is a fourth balance for crossover worlds, counted in whole
   dollars and shown as `$40`. It is gained, spent, given and awarded like a
   coin and has its own log entries, but it has no worth in coins: it cannot
-  be exchanged, a cash spend is paid from cash alone, and cash is never
-  broken to cover a coin spend. It shows in the purse only when held.
+  be exchanged on the sheet, a cash spend is paid from cash alone, and cash
+  is never broken to cover a coin spend. It shows in the purse only when
+  held.
+- **Gold and cash are changed for each other by an NPC**, not on the sheet.
+  An NPC the DM has given an exchange rate ("1 gold = $5") changes them
+  either way at that rate, in an NPC quest in progress that both the NPC and
+  the character are in. Only gold and cash: silver and copper are not taken.
+  Each NPC has their own rate. See [NPCS.md](NPCS.md), section 5.
+- **Buying.** In the same kind of scene a character can buy the weapons and
+  items an NPC, or the encounter itself, has for sale. The price is paid from
+  the purse like any other spend and logged as "Bought <thing> from <NPC>".
+  See [ENCOUNTERS.md](ENCOUNTERS.md), section 10.
 
 ### Giving to another character
 
@@ -748,6 +758,7 @@ Another Campaign" card on its sheet.
 | Level a weapon or item up or down | No | Yes | No |
 | Record a gold gain or spend | Yes | Yes | No |
 | Give gold, weapons, items or spells to another character | Yes | Yes | No |
+| Change gold and cash with an NPC, or buy what is for sale, in a quest in progress | Yes | Yes | No |
 | View the money log, gold and inventory | Yes | Yes | No |
 | Change level, XP, location or speed override | No | Yes | No |
 | Post a character note | Yes | Yes | Yes |
@@ -784,7 +795,7 @@ calculations for them.
 - Automatic levelling from XP
 - Alignment, languages, tool proficiencies and backgrounds-as-rules
 - Encumbrance and item weight
-- Item prices, cents, and an exchange rate between cash and coins
+- Cents, and changing silver or copper for cash
 
 ## 20. Everything the builder offers