Untitled

add_action( 'woocommerce_add_to_cart_validation', function( $is_allow, $product_id, $quantity ) {
	$product = get_post( $product_id );
	$product_author = $product->post_author;

	//Iterating through each cart item
	foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
		$cart_product_id = $cart_item['product_id'];
		$cart_product = get_post( $cart_product_id );
		$cart_product_author = $cart_product->post_author;
		if( $cart_product_author != $product_author ) {
			$is_allow = false;
			break;
		}
	}

	if( !$is_allow ){
		// We display an error message
		wc_clear_notices();
		wc_add_notice( __( "You already have products of a different seller in cart. You can only purchase from one seller at a time.", "marketking" ), 'error' );
	}
	
	return $is_allow;
}, 50, 3 );