I used to sell the chicks I hatched but not anymore. Luckily I haven't had any issues. I do sell hatching eggs in the spring but I do enclose a hatching egg disclaimer. I do test hatches prior to selling eggs to get an idea of the expected hatch rate and I always send extra eggs.
HATCHING EGG DISCLAIMER.
"There are no guarantees on hatching eggs. Please buy only if you are willing to risk that they may not hatch.
THIS IS DUE TO THE MANY FACTORS. THESE FACTORS INCLUDE BUT ARE NOT LIMITED TO:
-EXTREME TEMPERATURE CHANGES.
-ROUGH HANDLING OF THE PACKAGE DURING SHIPPING.
-X-RAYING THE PACKAGE BY THE POSTAL SERVICE.
-THE DISTANCE THE EGGS HAVE TO TRAVEL.
-THE INCUBATION PROCESS.
Although hatching eggs are assumed fertile, be aware that NO hatching eggs are guaranteed to hatch. There is a reasonable expectation for your eggs to arrive intact; not broken or cracked, and that they are actually fertile or at least EXTREMELY likely to be fertile. There is no way for breeders to test each egg for fertility prior to shipping, most fertile egg sellers take time to incubate & hatch from their own supply regularly to verify fertility. It is safe to assume that if their eggs are hatching, that all or most of the eggs will be fertile. When the eggs are gathered and mailed to you, the eggs may have contaminants on them. You should
wash your hands before and after handling the eggs. DO NOT WASH THE EGGS. You further accept there may be delays in mailing or processing. These are issues that may be beyond control and are an inherent risk of mailed hatching eggs. No matter how well any eggs are packed, you will probably never have a 100% hatch.
Understand that YOU, the buyer, bear ALL the responsibility for your purchase of hatching eggs!!!" Good luck and have fun...