Hi David,
You will find that eBay is such a competitive marketplace that you really won't have much control over setting your own margins, unless you have a product quite unique to any other seller and the demand for it is there from buyers. It's a matter of looking at the competition in your genre, then measuring that data against your product cost and other factors such as sell through rates and then that data can all be broken down to see where your product will sit in the market, and what margin can be made.
Cheers