I market for big ecommerce sites and I spend most of my days collecting and analyzing data and then testing out sales copy and promotions based on user engagement behavior patterns ... I just gave you a big tip; spend every waking second of your life collecting, compiling, analyzing, and testing data (sales psychology based) to be successful in this field. The best eBay sales generation would come from eBay itself. Your client's listings must be targeted correctly to the buyers. This does not mean copy and paste another person's sales page, this means to sit down study and test various pieces of sales copy until you find a solution that converts someone browsing into a buyer.
This is the difference between a true eBay powerseller and some idiot that just price cuts people and makes their "volume sales" that way.