Codisto does not support the eBay Sandbox environment. 

During testing we found inconsistent behaviour between our integration with the Live and Sandbox eBay environments, which would lead to a poor experience and the false impression that Codisto wasn't working correctly. So instead of working around those inconsistencies, we chose not so support the Sandbox, but instead implement functionality that made it's use unnecessary.

Preview

The most common reason we get asked about eBay Sandbox is to preview listings especially while editing templates.

It is easy to see the effect of template changes on the listing. Any product can be previewed via the  XpressGrid, even if it is not published as a listing. Simply select a product and click the Preview tab which pops out from the right of the XpressGrid. Any template edits made are reflected in Preview upon refresh.

Test Categories

Each eBay marketplace has a number of test categories where you can list products without worrying about them being sold (e.g. Lots More > Test Auctions. Just put "TEST LISTING" at the beginning of the title and description.