I've not sold duck eggs but I've been in business and self employed most of my life and gradually learned to make a good living at it. It takes time to build up reliable paying clients. Usually longer than you expected before you started. Craigslist or Facebook marketplace is a good place to begin as is the sign in the driveway. Be consistent with any advertising like Craigslist. Talk about it casually wherever you are. Don't push , just let people know you are raising ducks for eggs. Give out samples especially if you have extras that haven't sold but do so sparingly and attach a card with prices so people don't expect free. Post pictures of lovely food like this on your Facebook page and tell them how tasty your new duck eggs are. Some people use Instragram for pictures of ducks, eggs, egg dishes with a little description and pricing. If you keep at it, eventually you will have more customers than you can handle and will have to decide if you need to add more ducks.