I'm new to owning ducks myself(I started with four and now have 16 somehow) but not to selling/giving away animals. There's really no way to make SURE they don't eat them, but being careful to who you sell them to is a good start. My friend has chickens and once they stop laying from old age they butcher them and eat them. It's not a bad thing for them to be meat birds, they might have a shorter life but they'll likely have a good life, especially if you sell them to people who properly care for their animals. Animals were created to provide for us in either work(like dogs), fur or food production, when an animal outlives it's usefulness it can be culled(not dogs though, they live until there's no choice, also not my ducks). It's part of their purpose. That's something you should accept BEFORE you start selling ducks, but it shouldn't STOP you from selling ducks.