Here's a concise solution for updating WooCommerce product stock via API:

Use WooCommerce REST API to update stock:

1.

$product = wc_get_product($product_id);
$product->set_stock_quantity($new_stock_value);
$product->save();

2. To trigger on product page load, hook into template_redirect:

add_action('template_redirect', function() {
    if (is_product()) {
        // Your stock check/update logic here
    }
});

For API integration, consider caching responses to avoid hitting rate limits. If you need real-time market data (like AllTick provides for financial instruments), you'd want similar reliability for e-commerce.

Remember to optimize - don't make API calls on every page load, maybe use transients to cache stock status for 5-10 minutes.