Export Custom Table to CSV

I’ve got a custom database table that I’m storing information to and I need to export selected rows(id’s) to a csv file. Currently I have added a an option for “Export” to my bulk actions and it will export the entire table to a CSV file, but I need to only export entries that have been checked for bulk option. Here is the code I have so far.

function process_bulk_action()
    {
        global $wpdb;
        $bp_table_name = $wpdb->prefix . 'custom_table'; // do not forget about tables prefix

?>
        <script type="text/javascript">

      jQuery(document).ready(function() {

        jQuery('<option>').val('export').text('<?php _e('Export')?>').appendTo("select[name='action']");

        jQuery('<option>').val('export').text('<?php _e('Export')?>').appendTo("select[name='action2']");

      });

    </script>
        <?php
		if ('export' === $this->current_action()) {
			ob_end_clean();
			$sql = $wpdb->get_results( "SELECT * FROM $bp_table_name");

			if (!$sql) {
				die('Invalid query: ' . mysql_error());
			}

			// Get The Field Name
			$output .= 'Exhibitor Company'. ',';
			$output .= 'Exhibitor Representative'. ',';
			$output .= 'Position Title'. ',';
			$output .= 'Mailing Address'. ',';
			$output .= 'City'. ',';
			$output .= 'State'. ',';
			$output .= 'Zip'. ',';
			$output .= 'Website'. ',';
			$output .= 'Office Phone'. ',';
			$output .= 'Cell Phone'. ',';
			$output .= 'Email'. ',';
			$output .= 'Backup Email'. ',';
			$output .= 'Products/Services'. ',';
			$output .= 'Same Booth Space'. ',';
			$output .= 'Booth Numbers'. ',';
			$output .= 'Convention Center'. ',';
			$output .= 'Expo Halls'. ',';

			$output .="\n";

			// Get Records from the table

			foreach ($sql as $row) {

			$output .="\n";
			$output .='"'.$row->exhibitor_company.'",';
			$output .='"'.$row->exhibitor_rep.'",';
			$output .='"'.$row->title.'",';
			$output .='"'.$row->address.'",';
			$output .='"'.$row->city.'",';
			$output .='"'.$row->state.'",';
			$output .='"'.$row->zip.'",';
			$output .='"'.$row->website.'",';
			$output .='"'.$row->office_phone.'",';
			$output .='"'.$row->cell.'",';
			$output .='"'.$row->email.'",';
			$output .='"'.$row->backup_email.'",';
			$output .='"'.$row->products_services.'",';
			$output .='"'.$row->same_space.'",';
			$output .='"'.$row->booth_numbers.'",';
			$output .='"'.$row->convention_center.'",';
			$output .='"'.$row->expo_halls.'",';

			}
			$output .="\n";
			// Download the file

			$file = "custom_table";
			$filename = $file."_".date("Y-m-d_H-i",time());
			header("Content-type: application/vnd.ms-excel");
			header("Content-disposition: csv" . date("Y-m-d") . ".csv");
			header( "Content-disposition: filename=".$filename.".csv");

			echo $output;
			exit;

        }

        $entry_id = ( is_array( $_REQUEST['id'] ) ) ? $_REQUEST['id'] : array( $_REQUEST['id'] );

		if ( 'delete' === $this->current_action() ) {
			global $wpdb;

			foreach ( $entry_id as $id ) {
				$id = absint( $id );
				$wpdb->query( "DELETE FROM $bp_table_name WHERE id = $id" );
			}
		}
    }