When you try to buy a product you are taken to the checkout page but you are notified that your cart is empty. This problem is only happening when you are not logged in, when you are logged in the checkout page shows the product in the cart. Can you help me?
Alternatively , Disable all the plugin except WooCommerce and WC Marketplace for testing purposes and check again. If the issue still exists, please switch to some default theme like Twenty Sixteen and check again.
Let us know the result so we can help you out further.