.....
<td align="center">
   <a id="sell" onclick="forSale();"  href="<c:url value="item_delete.htm?id=${entry.key.id}"/>">Sell</a>
</td>
.....

function forSale() {
            var customer = prompt("Customer :", "");                    
        }

document.all["sell"]

function forSale(obj) {
            var customer = prompt("Customer :", "");
            obj.href = obj.href + '&customer=' + customer;
        }

<a id="sell" onclick="forSale(this);"  href="<c:url value="item_delete.htm?id=${entry.key.id}"/>">Sell</a>