Certified coins tend to bring more in the coin market, including eBay. There are two services I recommend: 1) PCGS (Professional Coin Grading Service) and 2) NGC (Numismatic Guaranty Corporation).
Good closeup photos that you can take with a digital camera are a must. Be sure to use the "macro" setting and no flash when you take the pics. Position your light source (a lamp is fine) off to the side somewhat of the coin.
Before you set your starting bid, see what coins like yours have sold for recently. Use the "Completed Listings" check box located on the left panel of the listings. Prices that show in green are for items that actually sold at those prices.
One more thing... Give some thought to your write-up. A gold coin is special. Learn something of the history behind these coins. Your twenty dollar gold piece, also known as a "double eagle", is much more interesting than a mere bullion coin. It doesn't hurt to romance the sale a bit! Check my web site for some ideas:
http://www.valuable-coin-stories.com/us-…