How long is a piece of string?
Impossible to tell in an online market such as eBay. Everyone's margin will be different, all depending on what the product is and what supplier price you are getting. In online sales, the cheaper you get a product, the more margin you have a chance of averaging, but you then have to factor competition into that equation to get your answer.
The tighter the competition, the lower your margin because everyone is trying to undercut each other to get the sale.