Don here from Shopify!
This sales channel is actually fully supported by eBay themselves, so there is not a lot we can tell you here in terms of troubleshooting. :)
Not to worry though, as there are dedicated support teams at eBay to help you out with this:
* Email: firstname.lastname@example.org
* Phone: 1 (855) 411-3376
Just get in touch with them directly and they should be able to help you resolve any issues with your eBay sales channel here.
All the best!