You assume that ebay chargers them the same rate they charge for small customers?
You also have to remember that selling in their stores costs them overhead.
Neither of those points answers his question. Even if ebay charges Target less than others it is still costing them something (as opposed to selling only on their own site).
As far as overhead goes. Why would there be lower overhead filling orders through ebay as opposed to just on their website (which theyre paying to maintain anyhow)?