Hello Eana,

One way to achieve this is to add the following PHP code snippet to your site:

// Add a filter to check the cart before proceeding to checkout
add_action( 'woocommerce_check_cart_items', 'check_cart_authors' );
function check_cart_authors() {
    // Initialize an empty array to store author IDs
    $author_ids = array();
    // Loop through each item in the cart
    foreach ( WC()->cart->get_cart() as $cart_item ) {
        // Get the author ID of the current item
        $author_id = get_post_field( 'post_author', $cart_item['product_id'] );
        // If the author ID is not in the array, add it
        if ( ! in_array( $author_id, $author_ids ) ) {
            $author_ids[] = $author_id;
        }
    }
    // If there is more than one author in the cart, show an error message and prevent checkout
    if ( count( $author_ids ) > 1 ) {
        wc_add_notice( __( 'You cannot checkout because the cart contains products from different vendors.', 'woocommerce' ), 'error' );
        // Remove the "Proceed to Checkout" button
        remove_action( 'woocommerce_proceed_to_checkout', 'woocommerce_button_proceed_to_checkout', 20 );
    }
}

It will show an error on the cart page if there are multiple vendors:

https://prnt.sc/QvNB_x5PKinf

The code can be added to functions.php or to any snippets plugin.

Also, to remove the standard message there saying “The products in your cart are sold by multiple different vendor partner….” you can go to MarketKing -> Settings -> Language & Text, and delete it.

Kind regards,

Thomas