Fertile and unfertile eggs are just the same for eating. Unless you are a experienced chicken keeper you will not even be able to tell the difference.
The "spot" you are referring to could possibly be a meat-spot, a small reddish spot in the egg, and is caused by a breakage of a blood vessel during the egg making process inside the hen.
They are perfectly safe to eat as well.
If your customers don't like them you can tell them why they are there and/ or to pick it out with a spoon.
The ones in the grocery store don't have them because they are candled with high power equipment to see any abnormalities inside the egg before packaging.
