eBayReady means that a product has all the details filled out so that it is ready to list on eBay. A product can be considered eBayReady even if it doesn't have any inventory and even if it isn't eBayEnabled yet.
On a product's eBayProperties page, you will see a green checkbox if it is eBayReady and a red X if it isn't. Hovering the mouse over the red X will display what fields are required in order for it to become eBayReady.
The required pieces of information are: