@startuml uc-07-trade-at-shop
autonumber
title UC-07 Trade at Shop (design sequence)
actor Player
participant "ShopBoundary\n«/api/characters/{id}/shop»" as boundary <<boundary>>
participant "ShopService\nsb-api::routes::{shop_action,buy,sell}" as control <<control>>
participant "ShopStock\nAppState.shop_stock" as entity <<entity>>

== View ==
Player -> boundary : GET shop
boundary --> Player : always-stocked + rotating stock, inventory

== Buy ==
Player -> boundary : POST buy + item id
boundary -> control : buy_item(character, item)
control -> entity : check stock (non-permanent: decrement)
alt stock exhausted
  entity --> control : 0 remaining
  control --> boundary : 410 GONE
else gold < price
  control --> boundary : 422
else ok
  control -> entity : stock -1, gold -price, inventory +item
  control --> boundary : {success, message, gold}
  boundary --> Player : {success, message, gold}
end

== Sell ==
Player -> boundary : POST sell + item id
boundary -> control : sell_item(character, item)
alt item not owned / unknown action
  control --> boundary : 400
else ok
  control -> entity : inventory -item, gold +value/2, stock +1 (non-permanent)
  boundary --> Player : {success, message, gold}
end
@enduml