The Upload option, when ran, instructs Tradebox to upload the Sage 'free stock' levels to the selected marketplace.
When upload is selected the above window will appear. Here you can choose whether to upload the stock levels as a CSV file or directly to the marketplace. The option to upload directly to the marketplace is only available for the listed marketplaces.
If a user has more than one sales channel of the same type, say two eBay sales channels, then the channel they wish to do the stock upload for can be selected from the Select Sales Channel list generated at the bottom of the window.
For eBay a video explaining the stock upload process can be viewed here.
A general troubleshooting guide for the stock upload process can also be viewed here.