I'd go with Ebay over Craigslist. There probably aren't that many people in your local area who are A) Looking for engagement rings, B) in the size you are selling, C) in the style you are selling, and D) in the price range you are selling, so Ebay will advertise to a much larger audience.
For what its worth, I bought my wife's engagement ring off Ebay (used, of course).