[Alex Monaghan]

You can get details of what has been sold via the Order API (triggered by the ION if required for real-time) and you can use the Product API to set stock levels.

I have a similar system working for another Ecwid customer, as long as you have a consistent SKU on both ends there's no reason why you can't keep systems in sync.

The only issue is that you can't update the Product API by SKU, only Ecwid ID so you'll need to map these yourself.