There are two solutions currently available. In the download section your find eBay Exporter which is a basic csv file generator that you can use to import products into Tubor Lister and then to eBay. Unfortunately, this mod has been abandoned for years but a great part of it still works and can be a simple start for implementation idea.

There's a commercial version in the Recommended Services section called AuctionBlox, it's a very expensive solution (in my opinion) and isn't updated enough to be in sync with Zen Cart. It's a great product when it works.