If you are intending to use Tradebox to upload stock levels to Bigcommerce and eBay, these two sites require an extra piece of information to be stored on the product record before the upload process can take place, namely the Listing or Product ID.
Using Sage Inventory
Once those steps have been completed, to add the Listing IDs to your eBay or Bigcommerce product records, go to Products in Tradebox and then select Import -> Products and choose either your Bigcommerce sales channel or your eBay sales channel from the drop down list.
Once the import is complete, the listing IDs will be added to the product record. You can view them by accessing the product record in the Products module and selecting the Channels tab:
Using Tradebox Inventory
If your inventory is set to Tradebox then you can download your products directly from eBay or Bigcommerce to create them in Tradebox. The menu can be accessed from the Products screen, by clicking Import -> Products and choosing either your eBay or Bigcommerce channel.
This will automatically create the product records and populate them with the Listing IDs. It will also automatically create a mapping between the online and Tradebox SKU.
If you've already created your products via another method, then you will need to ensure that product mappings are in place first, and select the Update Existing Records box on the Product Import options:
You can also tick the Update Quantities box to update the stock levels in Tradebox to match what's on Bigcommerce, if the levels in Tradebox have not been populated yet or are incorrect.