For WooCommerce the "note" field in the order contains the name of the marketplace the order came from, alongside the marketplace order number.
It will show under "Customer provided note" on the order screen.
For versions of WordPress version 4.4.0 or higher and WooCommerce version 2.6.0 or higher, you can also add the tag for example 'eBay' into a custom field of your choosing in
Omnivore under Settings -> Feeds -> General-> Order Settings-> "Marketplace channel field"
which can then be accessed as a distinct field in the order in WooCommerce.
While this is not necessarily visible in the WooCommerce order screens, it does mean that the attribute is inserted into the order metadata. This is used by some retailers to inform downstream order processing systems of the order's origin.