I sell my duck eggs for $15 per dozen which is an incredible value. You have to market them right to do that stating all of the benefits. Have a look at this website and see what they sell them for. 18 to 19 per HALF dozen. Do remember that ducks eat WAY more food than chickens. So they cost way more to raise. A study in Backyard Chicken magazine showed that Chickens cost $12 per dozen eggs when factoring and amortizing the cost of their shelter, the land they use, food, health care, etc. And that didn't include feeding your pets when they are too old to lay just because you like them and aren't going to kill them. With that considered the cost would be well over 12 per dozen. So don't feel like you are ripping anyone off because you're not. They sell them for $3 per dozen in the stores because they cram them in the shelter in terrible conditions and kill them immediately when they stop laying. I'm sure you're not doing that to your ducks.
Duck eggs last over 2 months in the refrigerator and over 2 weeks unrefrigerated.