diff --git a/brmbar3/brmbar-gui-qt4.py b/brmbar3/brmbar-gui-qt4.py
index 23e6d37..a351bee 100755
--- a/brmbar3/brmbar-gui-qt4.py
+++ b/brmbar3/brmbar-gui-qt4.py
@@ -1,6 +1,7 @@
 #!/usr/bin/python3
 
 import sys
+import subprocess
 
 from PySide import QtCore, QtGui, QtDeclarative
 
@@ -8,6 +9,15 @@ from brmbar import Database
 
 import brmbar
 
+# User credit balance limit; sale will fail when balance is below this limit.
+LIMIT_BALANCE = -200
+# When below this credit balance, an alert hook script (see below) is run.
+ALERT_BALANCE = 0
+# This script is executed when a user is buying things and their balance is
+# below LIMIT_BALANCE (with argument "limit") or below ALERT_BALANCE
+# (with argument "alert").
+ALERT_SCRIPT = "./alert.sh"
+
 
 class ShopAdapter(QtCore.QObject):
     """ Interface between QML and the brmbar package """
@@ -70,6 +80,18 @@ class ShopAdapter(QtCore.QObject):
         db.commit()
         return acct
 
+    @QtCore.Slot('QVariant', 'QVariant', result='QVariant')
+    def canSellItem(self, itemid, userid):
+        user = brmbar.Account.load(db, id = userid)
+        if -user.balance() > ALERT_BALANCE:
+            return True
+        elif -user.balance() > LIMIT_BALANCE:
+            subprocess.call(["sh", ALERT_SCRIPT, "alert"])
+            return True
+        else:
+            subprocess.call(["sh", ALERT_SCRIPT, "limit"])
+            return False
+
     @QtCore.Slot('QVariant', 'QVariant', result='QVariant')
     def sellItem(self, itemid, userid):
         user = brmbar.Account.load(db, id = userid)
@@ -99,6 +121,11 @@ class ShopAdapter(QtCore.QObject):
         db.commit()
         return balance
 
+    @QtCore.Slot('QVariant', result='QVariant')
+    def balance_user(self, userid):
+        user = brmbar.Account.load(db, id=userid)
+        return user.negbalance_str()
+
     @QtCore.Slot(result='QVariant')
     def balance_cash(self):
         balance = shop.cash.balance_str()
diff --git a/brmbar3/brmbar-gui-qt4/ItemInfo.qml b/brmbar3/brmbar-gui-qt4/ItemInfo.qml
index a39e322..11e959d 100644
--- a/brmbar3/brmbar-gui-qt4/ItemInfo.qml
+++ b/brmbar3/brmbar-gui-qt4/ItemInfo.qml
@@ -58,6 +58,8 @@ Item {
             if (acct.acctype == "cash") { //Copied from BarButton.onButtonClick
                 shop.sellItemCash(dbid)
                 status_text.setStatus("Sold! Put " + price + " Kč in the money box.", "#ffff7c")
+            } else if (!shop.canSellItem(dbid, acct.id)) {
+                status_text.setStatus("NOT SOLD! "+acct.name+"'s credit is TOO LOW: "+shop.balance_user(acct.id), "#ff4444")
             } else {
                 var balance = shop.sellItem(dbid, acct.id)
                 status_text.setStatus("Sold! "+acct.name+"'s credit is "+balance+".", "#ffff7c")