Restrict WC Vendors purchasing own products

I suppose that your “post authors” (vendors) have a custom role (or some specific capabilities). So you can target the user role “post authors” (or one of his specifics capabilities) checking and removing cart items on checkout page for this user roles.

Here is that code:

add_action( 'woocommerce_before_checkout_form', 'checking_allowed_cart_items_on_checkout', 100, 1);
function checking_allowed_cart_items_on_checkout() {

    // Allow only "administrator" and "customer" user roles to buy all products
    if( ! current_user_can('administrator') || ! current_user_can('customer') ){

        // Ititializing variables
        $found = false;
        $cart_object = WC()->cart;
        $current_user_id = get_current_user_id();

        // Checking cart items
        foreach($cart_object->get_cart() as $cart_item_key => $cart_item){
            // get the WC_Product object and the product ID
            $product = $cart_item['data'];
            $product_id = method_exists( $product, 'get_id' ) ? $product->get_id() : $product->id;

            // Get the product post object to get the post author
            $post_obj = get_post( $product_id );
            $post_author = $post_obj->post_author;

            if( $post_author == $current_user_id ){
                // removing the cart item
                $cart_object->remove_cart_item( $cart_item_key );
                $found = true; // set to true if a product of this author is found
            }
        }

        // Display a custom message in checkout page when items are removed
        if( $found ){
            // Display an error message
            wc_add_notice( __( 'Items removed - As a Vendor, you are not allowed to buy your own products.' ), 'error' );
        }
    }