I agree with Oregon Blues (often). I am not sure that selling duck eggs is a profitable endeavor. I am a professional baker, and prefer duck eggs for baking. I also use a lot of duck eggs, and prefer fresh, local food. I raise my own ducks to support my habit, and not have to drive to market twice a week. I also like being out there with them, and there is something relaxing in the routine of their care.
I think, if selling is a goal, being able to do so at farmers markets is a good bet. Your eggs will likely have to cost a bit more, to cover the certification process, but at markets I think people are more willing to pay a higher price.
Another good avenue you may want to try - bakeries & restaurants.
With an ever growing portion of the population asking where their food comes from more restaurants are looking for locally sourced products (FINALLY!!!!!). I know many chefs, bakers & restauranteurs who would be thrilled to have farm fresh eggs come to them. So few of us get to leave the kitchen for anything. The farmers who come to us are kept pretty busy with deliveries once they prove reliable.
If you decide to go this route a word of advice - chefs say no to everything during busy times. I would go between the hours of 1 & 3:30 or early - 10 am, to contact them. I would adhere to those hours with phone calls, too. We remember the person who shows up during a lunch or dinner rush (6-9pm usually), and not fondly. Email is good, but I would do an in person visit regardless of whether or not you get a response.