How to add a select menu to this metabox code?

Basically you need to add the html of the select dropdown at your function inventory_information() which is the function that actually displays the metabox:

// The Event Location Metabox

        function inventory_information() {
            global $post;

            // Noncename needed to verify where the data originated
            echo '<input type="hidden" name="inventorymeta_noncename" id="inventorymeta_noncename" value="' .
            wp_create_nonce( plugin_basename(__FILE__) ) . '" />';

            // Get the location data if its already been entered
            $stocknum = get_post_meta($post->ID, '_dappcf_i_stocknum', true);
            $vin = get_post_meta($post->ID, '_dappcf_i_vin', true);
            $saleprice = get_post_meta($post->ID, '_dappcf_i_priceone', true);
            $internetprice = get_post_meta($post->ID, '_dappcf_i_pricetwo', true);
            $milage = get_post_meta($post->ID, '_dappcf_i_mileage', true);
            $carfaxurl = get_post_meta($post->ID, '_dappcf_i_carfaxurl', true);

            //here you add the dropdown as value if already set so you add something like
            $my_dropdown = get_post_meta($post->ID, '_dappcf_i_dropdown', true);

            // Echo out the fields
            echo '<p>Stock #: <input type="text" name="_dappcf_i_stocknum" value="' . $stocknum  . '" class="widefat" style="width:80px" /> &nbsp;&nbsp;&nbsp; 
                Milage: <input type="text" name="_dappcf_i_mileage" value="' . $milage  . '" class="widefat"  style="width:80px" /> &nbsp;&nbsp;&nbsp; 
                VIN: <input type="text" name="_dappcf_i_vin" value="' . $vin  . '" class="widefat"  style="width:200px" />';
            echo '<p>Sale Price: <input type="text" name="_dappcf_i_priceone" value="' . $saleprice  . '" class="widefat"  style="width:80px" /> &nbsp;&nbsp;&nbsp; 
                Internet Price: <input type="text" name="_dappcf_i_pricetwo" value="' . $internetprice  . '" class="widefat"  style="width:80px" />';
            echo '<p>CarFax url: <input type="text" name="_dappcf_i_carfaxurl" value="' . $carfaxurl  . '" class="widefat"  style="width:170px" />';

            //here you add the HTML of the dropdown you add something like
            echo '<p>Select menu: <select name="_dappcf_i_dropdown" class="widefat">';
            echo '<option value="1"'. $my_dropdown == "1" ? ' selected="selected"' : ''. '>' . 'Option 1'. '</option>';
            echo '<option value="2"'. $my_dropdown == "2" ? ' selected="selected"' : ''. '>' . 'Option 2'. '</option>'; 
            echo '<option value="3"'. $my_dropdown == "3" ? ' selected="selected"' : ''. '>' . 'Option 3'. '</option>'; 
            echo '<option value="4"'. $my_dropdown == "4" ? ' selected="selected"' : ''. '>' . 'Option 4'. '</option>'; 

            //add as many as you need here

            echo '</select>';
        }

and then just add that field to your metabox save function which is txpbs_save_events_meta so:

// Save the Metabox Data

        function txpbs_save_events_meta($post_id, $post) {

            // verify this came from the our screen and with proper authorization,
            // because save_post can be triggered at other times
            if ( !wp_verify_nonce( $_POST['inventorymeta_noncename'], plugin_basename(__FILE__) )) {
            return $post->ID;
            }

            // Is the user allowed to edit the post or page?
            if ( !current_user_can( 'edit_post', $post->ID ))
                return $post->ID;

            // OK, we're authenticated: we need to find and save the data
            // We'll put it into an array to make it easier to loop though.

            $station_meta['_dappcf_i_stocknum'] = $_POST['_dappcf_i_stocknum'];
            $station_meta['_dappcf_i_vin'] = $_POST['_dappcf_i_vin'];
            $station_meta['_dappcf_i_priceone'] = $_POST['_dappcf_i_priceone'];
            $station_meta['_dappcf_i_pricetwo'] = $_POST['_dappcf_i_pricetwo'];
            $station_meta['_dappcf_i_mileage'] = $_POST['_dappcf_i_mileage'];
            $station_meta['_dappcf_i_carfaxurl'] = $_POST['_dappcf_i_carfaxurl'];

            //here just add the dropdown field to the $station_meta array from the $_POST
            $station_meta['_dappcf_i_dropdown'] = $_POST['_dappcf_i_dropdown'];


            // Add values of $station_meta as custom fields

            foreach ($station_meta as $key => $value) { // Cycle through the $station_meta array!
                if( $post->post_type == 'revision' ) return; // Don't store custom data twice
                $value = implode(',', (array)$value); // If $value is an array, make it a CSV (unlikely)
                if(get_post_meta($post->ID, $key, FALSE)) { // If the custom field already has a value
                    update_post_meta($post->ID, $key, $value);
                } else { // If the custom field doesn't have a value
                    add_post_meta($post->ID, $key, $value);
                }
                if(!$value) delete_post_meta($post->ID, $key); // Delete if blank
            }

        }

Hope this helps.