ปัญหาของ cart เมื่อสินค้าเป็น0ชิ้น ทำไมถึงสั่งซื้อได้ แก้ยังงัยแนะนำหน่อยครับ มีโค๊ด

<?
include"connect.php";

 	$sum=0;
 		for($i=0;$i<=50;$i++)
	{
		if($buy[$i]!="")
		{
			$cmd = "select * from product where pro_auto = '$buy[$i]'";
			$result = mysql_query($cmd,$link);
			$row=mysql_fetch_array($result); 
		?>
		<tr bgcolor="#B2DFFF">
  	 	   	<td align="center" bgcolor="#FFFFFF"><? echo"<font size='2'>$row[pro_id]";?> </td>
  		<td align="center" bgcolor="#FFFFFF"><? echo"<b><font color = '#FF5FAA'>$row[pro_name]</b>";?> </td>
  		<td align="center" bgcolor="#FFFFFF"><? echo"$row[pro_pricesale] "; ?> </td>
  		<td align="center" bgcolor="#FFFFFF"><? echo"$row[7] </font>"; ?> </td>
		<td align="center" bgcolor="#FFFFFF">
		<?
			$num_pros = mysql_num_rows(mysql_query("Select * from productstock where pro_auto='$row[pro_auto]' and pros_status='static' "));
		 	echo"$num_pros </font>"; 
		 ?>			</td>
  		<td align="center" bgcolor="#FFFFFF"> 
 			<?	
					//$bn=$buy[$i];echo $bn." ".$buytemp[$i];
					if($num_pros>0){
					echo "<input type ='text' name = 'temp$i' size='2' onchange='javascript:if(checknum2(document.acart.temp$i.value,$row[0],$i) == false) { document.cart.temp$i.value=\"$buytemp[$i]\"; } ' value='$buytemp[$i]' > ";
					}else {
					echo "<input type ='text' name = 'temp$i' size='2' readonly='true'> ";
					}
		?>			</td>
  		<td align="center" bgcolor="#FFFFFF"><? echo"<a href ='cart?delbuy=$i'><img src='IMG/delete.gif' border='0'></a>"; ?> </td>
  	    <? $m++; ?>
           <?
				$sum=$sum+($row[pro_pricesale]*$buytemp[$i]);
				setcookie("sale_cost","$sum");
				
			}
		}						
	?>
 	      </form>
      </tr>	 
 	    </table>		
    <p>
      <?
   echo"<p align='center'><font color = 'yellow'>Total&nbsp;&nbsp;<b>$sum</b>&nbsp;&nbsp;bath : </font>( <a href= 'cart'><img src='IMG/calculator.gif' border='0' align='absmiddle'> คำนวนใหม่</a> )<Br><BR> [-- <a href= 'cart?action=yesbuy'><img src='IMG/money.gif' border='0' align='absmiddle'> สั่งซื้อสินค้า</a> --]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[-- <a href='index.php'><img src='IMG/redo.gif' border='0' align='absmiddle'> กลับไปซื้อต่อ</a> --]</p> ";?>