The search is stored in a session variable.
Adda link to the page like:
<a href="buynintendodsgames.php?reset=true">Clear Search</a>
Then add the following around line 1:
<?php
if(!session_id()) session_start();
if(isset($_SESSION["WADbSearch1_sellnintendodsgames"]) && isset($_GET['reset']) && $_GET['reset'] == "true") {
unset($_SESSION["WADbSearch1_sellnintendodsgames"]);
}
?>