Fertile 5 month old Khaki Campbell drake?

[Realtree4]

My khaki Campbell's were hatched out mid-March and I have 1 male and 3 females.....which have all been laying a month(ish). I have seen the bullseye in the eggs so I "think" they're fertile?

The reason I'm asking is someone wants to buy hatching eggs from me and I'm such a newbie with that! Other than having a broody chicken and letting her do all the work, I have no experience in hatching eggs. The size of the eggs are the slightly larger than a store bought chicken egg, I just don't know if they're large enough yet to hatch out ducklings? And what age are drake usually to get a good hatch rate? He doesn't seem to have a difficult time mating!

 

[Blue Rooster]

Duck eggs are a lot larger that chicken eggs. Two duck eggs equal 3 chicken eggs.
To be sure that the eggs are large/strong enough i would wait about a month and then hatch some out. And as long as your drake is mating with your females i think the fertility rate will be high.

And a white dot/bulls eye means the egg is fertile.