Can I re-open the checkout for an eBay order?

No. Because eBay processes orders and payment through their own checkout, it is not possible for us to alter the eBay orders that we import. Any alterations would cause issues with the integration between the two systems, resulting in errors in your Message Center and the inability to properly update the order at eBay with shipping/tracking information. 

If you have an eBay buyer interested in purchasing additional items, please direct them to purchase them from eBay or create a new order within CA using the Checkout Direct method: http://ssc.ChannelAdvisor.com/support/faqs/how-can-i-create-manual-order-ca