
  No one knows until they crack it open, the outside of a fertile egg is exactly the same as a non-fertile one, and as I said, most people honestly have no idea what to look for anyway.  They just have visions of partially formed chicks coming out. 
If you are selling eggs then you have to do what you have to do.  I might just have a conversation with those customers to see what the actual objection is and who knows, maybe they will change their mind.  Maybe not.  I know people whom have told me the same thing and when questioned (politely) they say basically 'because someone told me', they really have no idea.  You could always have a flock of hens separate and a flock with a male, and do both.  Just depends on what your goals and needs are.