The first thing that you need to do is experiment with backgrounds. Dark backgrounds tend to work best for shiny coins, especially polished gold and silver coins. But, some coins do look great against white backgrounds.

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It may be best to buy a pack of multi-colored paper from your local store and set a few coins against a variety of backgrounds. Take a few shots and see which backgrounds work best for your particular coins.
Next, you want to create some shadows so that your coin doesn’t just look a shiny object with no definition. Each coin has some type of stamping on it. You want to show this off in your eBay ad. One way to create shadows is by adding an outside light source. A simple flashlight that won’t roll around (i.e. one with a clamp or one that you can clamp down with something else) will work fine.
Here is another opportunity to do some experimentation. Most shadows will pop out from a side angle, but you should position the light at differing angles, take a few shots and see which angle works best. The light should be positioned maybe a foot or two away from your coin so that it doesn’t over-expose the coin. You want to create shadows, not more shiny spots.