Pure bred stock will probably be easier to sell, especially hatching eggs. People hatching eggs like to know what to expect. If you only have one breed, it's very easy to avoid unwanted crosses. With more than one breed you need to pen them seperately for as long as you're collecting eggs to sell for hatching. And you'll need to have them seperated for at least 10 days after seperating them to avoid accidental crosses. They will lay fertile eggs most of the year. Eggs you aren't going to hatch can be sold as food, but you're better off just selling hatching eggs. You can sell hatching eggs for 10x what they will sell for as food. I also use my extras for baking. The problem with selling them as food is they'll still be fertile so you might be undercutting your own prices.
You'll probably want to sell adult ducks too at some point. If you're breeding the best quality ducks you can you'll need to be selective about your breeding stock and improve it by buying from other breeders, mixing your bloodlines within the breed, and culling your least desirable ducks.
Whatever breeds you decide on you'll need to make sure there's a market for them. I have Ancona and Swedish. I like the Ancona and they're a threatened breed in the Livestock Conservancy list. I added Swedish because they're endangered too, are similar to Ancona, and are rare in my part of the country.
If you check out the Conservancy website you can see what breeds are popular, which are rarer, and where you can find eggs or ducklings. I'm not really interested in selling ducks, geese, chickens, or eggs, but still know the more uncommon something is the easier it is to generate demand. Breeders like Greenfire specialize in rare breeds because it's easier than competing against larger commercial hatcheries selling common breeds.
If your niche is going to be three breeds of duck you'll need to plan for internet sales, shipping, and marketing on top of whatever local/direct sales you're planning. That's going to involve some research into what your state, county, and municipality requires as well as what you can do to cut some of your overhead costs.
It's fine to turn your hobby into a business, but try not to lose sight of the fact that it's a hobby because it's interesting and fun. If the business isn't fun or interesting it will be time to get a new hobby.
