Before listing your products to eBay or Amazon you will need to create Listing Templates. This will tell the channel where to list the products associated with that template, along with the shipping information and, in the case of Amazon, the Size or Colour mapping as well as category specific specifications.
This guide will show you how to create a listing template for eBay.
This guide will show you how to create a listing template for Amazon.
Once you have created your template(s), you will be ready to list to either (or both) channel.