Hi @SolventDirectGB .
You may want to try Serializer - Product Tracking. With the app, you can add searchable serial numbers to your sold items. If your download links are dynamic and can’t be inferred by serial numbers alone, you could also associate the item with the download link via the app’s “Custom fields” feature.
The app provides a searchable API. However, a little custom development would be required in your theme to hook up the API with the visuals on your website, e.g. search input field and API call handling.