Q: I bought a dozen eggs and many of the eggs in the carton had blood spots. Can I demand a new carton of eggs to replace them?
A: You should be entitled to return the eggs for replacements, since eating eggs with blood spots is prohibited. However, the Shulchan Aruch writes that nowadays the practice is not to return them, and that in this context the common practice (minhag) supersedes halacha (Choshen Mishpat 232:19). This is because the laws of buying and selling depend on the understanding between the involved parties, and everybody buys and sells based on the common commercial practice.
Similarly, if an animal is slaughtered and found to be treif, the Shulchan Aruch rules that the seller has to refund the money (Choshen Mishpat 232:11). However, Pischei Teshuva writes that apparently the custom nowadays is not so and the custom prevails, so that Beis Din has to verify the local practice in each place (Choshen Mishpat 232:6).
This principle is significant in other situations where the common commercial practice is at variance with the standard halacha, as we will see in the next issue.