Index: docs/P3-UseCaseModel/UseCase0005.md
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+# Use-case 0005 — Place market SELL order
+
+**Initiating actor:** Trader
+
+**Other actors:** Market Simulator (indirect — supplies the current price).
+
+A Trader sells part or all of a holding at the current market price. Cost basis is preserved so realised P/L can be reconstructed from the ledger.
+
+## Scenario
+
+1. Trader chooses "Place market SELL order".
+2. System lists markets (same SQL as UC0004 step 2).
+3. Trader enters market symbol and quantity.
+4. System resolves the market and looks up the latest price (same SQL as UC0004 step 4).
+5. System opens a transaction:
+
+   ```sql
+   BEGIN;
+
+   INSERT INTO project.orders
+       (user_id, market_id, side, type, status, quantity, price, executed_at)
+   VALUES
+       ($user_id, $market_id, 'sell', 'market', 'executed', $qty, $price, now())
+   RETURNING id;   -- $order_id
+
+   SELECT quantity, avg_price
+     FROM project.holdings
+    WHERE user_id = $user_id AND crypto_id = $crypto_id
+    FOR UPDATE;
+   -- abort if row missing or quantity < $qty
+   ```
+6. If the holding check passes, system reduces the holding, credits cash and debits invested, and appends a ledger and a market trade:
+
+   ```sql
+   UPDATE project.holdings
+      SET quantity   = quantity - $qty,
+          updated_at = now()
+    WHERE user_id = $user_id AND crypto_id = $crypto_id;
+
+   UPDATE project.users
+      SET available_balance = available_balance + $notional,
+          invested_balance  = GREATEST(invested_balance - ($avg_price * $qty), 0),
+          updated_at        = now()
+    WHERE id = $user_id;
+
+   INSERT INTO project.transactions
+       (user_id, type, amount, currency, related_order, description)
+   VALUES
+       ($user_id, 'sell', $notional, 'USD', $order_id, 'Market sell ...');
+
+   INSERT INTO project.market_trades
+       (market_id, executed_at, price, quantity, side, source)
+   VALUES
+       ($market_id, now(), $price, $qty, 'sell', 'user');
+
+   COMMIT;
+   ```
+7. System confirms: `Order executed: sell 0.5000 ETH @ 3520.000000 (notional 1760.0000 USD)`.
+
+### Alternate flow 5a — insufficient holding
+
+If the `SELECT ... FOR UPDATE` returns no row, or the held quantity is smaller than the sell quantity, the entire transaction rolls back and system shows "Insufficient holding: trying to sell X, hold Y."
+
+### Realised P/L (post-scenario)
+
+The realised P/L for a sell is `$notional - ($avg_price * $qty)`. It is not persisted explicitly but can be computed from the ledger and the holding at sell time.