
As I see this, you would need to write your listings on your own site so that they can be copied to Auctiva or eBay. You would need a working relationship with them like between Auctiva and eBay.
There will be a defined interface between source and destination and a business arrangement would need to be set up between you and whoever the listings were transferred to or from.
Then if people buy from your site you need a safe payment system, do we have a rival germinating here ?