Seems to work perfectly. Only one problem - when I add 5 items to market it reaches the limit and I need to wait for others to buy it or remove item from market. But I've noticed that it doesn't update my marketquota after someone buys my item or I remove it. So i've updated the code a little bit. 
Find this somewhere around line 40: 
 $MONEY=$r['imMONEY']*$_POST['rqty'];
         $CRYSTALS=$r['imCRYSTALS']*$_POST['rqty'];
         mysql_query("UPDATE itemmarket SET imQTY=imQTY-{$_POST['rqty']} WHERE imID=".abs(intval($_GET['ID']))." AND imADDER=".abs(intval($ir['userid']))."",$c);
         mysql_query("DELETE FROM itemmarket WHERE imQTY=0",$c);
  
ADD BELOW: 
  
 if ($r['imQTY'] == $_POST['rqty']) {
	  mysql_query("UPDATE users SET marketquota=marketquota-1 where userid=".abs(intval($r['imADDER']))."",$c);
	  }
-------------------------------------------------- 
Find this few lines below: 
  
item_add($userid, $r['imITEM'], 1);
 $i = mysql_insert_id($c);
 mysql_query("DELETE FROM itemmarket WHERE imID=".abs(intval($_GET['ID']))."",$c);
  
ADD BELOW: 
  
mysql_query("UPDATE users SET marketquota=marketquota-1 where userid=".abs(intval($r['imADDER']))."",$c);
-------------------------------------------------- 
Find thins in around line 120: 
 item_add($userid, $r['imITEM'], $_POST['buyqty']);
 mysql_query("UPDATE itemmarket SET imQTY=imQTY-".abs(intval($_POST['buyqty']))." WHERE imID=".abs(intval($_GET['ID']))."",$c);
 mysql_query("UPDATE users SET money=money-$MONEY, crystals=crystals-$CRYSTALS WHERE userid=".abs(intval($userid))."",$c);
 mysql_query("UPDATE users SET money=money+$MONEY, crystals=crystals+$CRYSTALS WHERE userid=".abs(intval($r['imADDER']))."",$c);
  
ADD BELOW: 
  
if ($r['imQTY'] == $_POST['buyqty']) {
	  mysql_query("UPDATE users SET marketquota=marketquota-1 where userid=".abs(intval($r['imADDER']))."",$c);
	  }
-------------------------------------------------- 
And finally, find this around line 165: 
item_add($userid, $r['imITEM'], 1);
 $i=mysql_insert_id($c);
 mysql_query("UPDATE users SET money=money-{$r['imMONEY']}, crystals=crystals-{$r['imCRYSTALS']} where userid=".abs(intval($userid))."",$c);
 mysql_query("UPDATE users SET money=money+{$r['imMONEY']}, crystals=crystals+{$r['imCRYSTALS']} where userid=".abs(intval($r['imADDER']))."",$c);
  
AND ADD: 
  
mysql_query("UPDATE users SET marketquota=marketquota-1 where userid=".abs(intval($r['imADDER']))."",$c);
-------------------------------------------------- 
Should work now.