How to make a metabox that contains link categories?

Found the last bit. It was just a syntax error:

<?php selected( $selected, ".$term_id". ); ?>

Should be…

<?php selected( $selected, $term_id ); ?>

Entire Function:

function tf_book_deets_create(){
add_meta_box('tf_book_purchase', 'Book Purchase Links', 'tf_book_purchase', 'books','side','default');
}
function tf_book_purchase (){
    global $post;
            $custom  = get_post_custom($post->ID);
            $link    = $custom["link"][0];
            $selected = isset( $custom['link'] ) ? esc_attr( $custom['link'][0] ) :'';


            echo '<div class="link_header">';

            $myterms = get_terms("link_category");

             echo '<p>Please select a set of purchase links for this book.</p>';

            echo '<select name="link" id="link">';
            echo '<option class="buy_books">Select A Link Category</option>';
            foreach($myterms as $term){
                        $term_slug=$term->slug;
                        $term_name =$term->name;
                        $term_id =$term->term_id;


           ?>   <option value="<?php echo $term_id;?>" <?php selected( $selected, $term_id ); ?>><?php echo $term_name;?></option>


           <?php   } echo '</select><br /></div>';


}



    add_action ('save_post', 'save_tf_book_purchase');

    function save_tf_book_purchase() {

    global $post;

    // make sure we're on a supported post type
        if ( $_POST['post_type'] != 'books' ) return;  

        // verify this came from our screen and with proper authorization.
        if ( !wp_verify_nonce( $_POST['book_nonce_name'], 'book-nonce' )) return;

        // verify if this is an auto save routine. If it is our form has not been submitted, so we dont want to do anything
        if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) return;


        // Check permissions
        if ( 'page' == $_POST['post_type'] ) {
            if ( !current_user_can( 'edit_page', $post_id ) ) return;
        } else {
            if ( !current_user_can( 'edit_post', $post_id ) ) return;
        }

    //if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE){ return $post->ID; }

    update_post_meta( $post->ID, 'link', esc_attr( $_POST['link'] ));

    }