How to give new users two specific user role options upon WordPress user registration

see register_form action hook for modifying the registration form

add_action( "register_form", function() {
    $html="<p>".__('Do you want to buy or sell?<br>Choose your role').'</p>';
    $html .= '<p>';
        $html .= '<label for="_customer"><input type="radio" name="user_role" id="_customer" value="customer"/>'.__('Customer').'</label><br>';
        $html .= '<label for="_vendor"><input type="radio" name="user_role" id="_vendor" value="_vendor"/>'.__('Customer').'</label>';
    $html .= '</p>';
    echo $html;
} );

you can use user_register action to save selected user role.

add_action( "user_register", function( $user_id ) {
    $allowed = array( 'customer', 'vendor' );
    if( isset( $_POST['YourFieldName'] ) && in_array( $_POST['YourFieldName'], $allowed ) ){
        $user = new WP_User( $user_id );
        $user->set_role($_POST['YourFieldName']);
    }
} );

check WP_User class reference document for more information;