Hello othreed,
Not sure if this is exactly what you're looking for, but I have developed a module that allows you to create custom invoice text messages by customer group.
If you can assign the ebay customer to an ebay customer group then the module may help you.
More information and screenshots of the module can be found here:
https://www.logicalit.com/prestashop-modules/en/6-invoice-text-by-customer-group-module.html