Paypal ipn catch payment cancel event - paypal

Question

I am trying to do the following: When the user selects some items and click "Pay Now" button on our website, I mark the selected items status as "on hold" so no one else can select these items. If I receive "complete" from ipn I will mark the items as "purchased" and do other database change. If I receive "canceled" from ipn I will mark the "on hold" items back to "available".
It works fine when the user did make the payment (complete). However, if the user exit the payment flow (for example just close the browser tab) I didn't receive any messages. I am not sure why this is happening. Or the ipn just wouldn't give a message if the transaction is not complete?
Here is the code of the paypal button:
<form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="business" value="********-facilitator#gmail.com">
<input type="hidden" name="env" value="www.sandbox">
<input type="hidden" name="address_override" value="1">
<c:set var="index" value="${1}"/>
<c:forEach items="${items}" var="item">
<input type="hidden" name="item_name_${index}" value="${item.title}">
<input type="hidden" name="amount_${index}" value="${item.askPrice}">
<input type="hidden" name="tax_${index}" value="$<fmt:formatNumber type="number" maxFractionDigits="2" value="${item.askPrice * taxRate}" />">
<c:if test="${index == 1}">
<input type="hidden" name="shipping_${index}" value="$<fmt:formatNumber type="number" maxFractionDigits="2" value="${shipmentFee}" />">
</c:if>
<c:set var="index" value="${index + 1}"/>
</c:forEach>
<c:remove var="index"/>
<!-- Fill full name in the first_name field -->
<input type="hidden" name="first_name" value="${info.recipient}">
<input type="hidden" name="notify_url" value="<c:url value="https://********.localtunnel.me/payment/paypal/ipn"></c:url>">
<input type="hidden" name="address1" value="${info.addressLine1}">
<input type="hidden" name="address2" value="${info.addressLine2}">
<input type="hidden" name="city" value="${info.city}">
<input type="hidden" name="state" value="${info.state}">
<input type="hidden" name="zip" value="${info.zip}">
<input type="hidden" name="country" value="US">
<input type="hidden" name="email" value="${info.email}">
<input type="hidden" name="custom" value="${flowId}">
<input type="image" name="submit" border="0"
src="https://www.paypalobjects.com/webstatic/en_US/i/buttons/checkout-logo-large.png" alt="Check out with PayPal">
</form>