fixed #8

src/main/java/ua/leonidius/trdinterface/sell/Sell.java

@@ -22,7 +22,12 @@ public static void sellItem(Player player, String id, int amount) {
 
         double price = Trading.sellCfg.getDouble(id.replace(":", "-"));
 
-        double cost = price*amount;
+        double cost = price * amount;
+
+        if (!player.getInventory().contains(item)) {
+            player.showFormWindow(new SellFailScreen());
+            return;
+        }
 
         player.getInventory().removeItem(item);
         EconomyAPI.getInstance().addMoney(player, cost);

src/main/java/ua/leonidius/trdinterface/sell/SellFailScreen.java

@@ -0,0 +1,19 @@
+package ua.leonidius.trdinterface.sell;
+
+import cn.nukkit.event.player.PlayerFormRespondedEvent;
+import cn.nukkit.form.element.ElementButton;
+import cn.nukkit.form.window.FormWindowSimple;
+import ua.leonidius.trdinterface.Message;
+import ua.leonidius.trdinterface.screens.Screen;
+
+public class SellFailScreen extends FormWindowSimple implements Screen {
+
+    public SellFailScreen() {
+        super(Message.WDW_FAIL_TITLE.getText(), Message.WDW_SELL_NOTHING.getText());
+        addButton(new ElementButton(Message.BTN_BACK.getText()));
+    }
+
+    public void onResponse(PlayerFormRespondedEvent event) {
+        event.getPlayer().showFormWindow(new SellItemSelectorScreen(event.getPlayer().getInventory()));
+    }
+}