Why would an order get cancelled?

If an Amazon, eBay or Walmart order which has been sent to your e-commerce platform gets cancelled on the marketplace, the corresponding order in your e-commerce platform is cancelled.

What happens to inventory levels?

Inventory levels remain unaffected. The cancellation does not restock the product. This behaviour is deliberate. Consider the possible reasons for a cancellation, for example, unable to fulfil due to soiled or missing stock. The only exception to this is if the order is cancelled before it is paid, as the cancellation is due to non-payment, so the product is restocked in this scenario.

We don't get the reason from the marketplace, only the fact it was cancelled. If Codisto was to restock the product, that risks overselling of an item which is unavailable. 

If an order is cancelled due to a customer reason, not a merchant reason, please manually adjust the inventory to put the product back into stock.