First of all try to sell it locally: community newspaper, Mac users group, university. Little risk on a cash, face to face deal.
The shipping on an eMac sold via eBay will be significant, and will reduce your closing price; and on eBay or Craigslist, you have to cover your #$$ when it comes to fraudulent buyers, and give both eBay and PayPal a % cut.
BTW, I had great luck selling my Indigo iMac DV on craigslist a few months ago. I had bought it on eBay and actually got more for it than I paid! I bought it for $130 shipped and I sold it for $175 2 months later. Whee.