Long ago I met a Jewish couple whose business was selling secular Christmas ornaments and paraphernalia. Is there universal agreement on whether this is halachically allowed?
Verbally declaring products as "Christ-mas" ornaments references the name of an idol (established in Romans 9:5) that is not mentioned in Torah. | Selling products verbally by announcing them as "Christ-mas" items would violate Shemot 23:13 based on [ Sanhedrin 63b.5-8 ].
"It is prohibited for a person to enter into a partnership with a gentile, lest their joint ventures lead them to quarrel, and his gentile partner will be obligated to take an oath to him, and he will take an oath in the name of his object of idol worship and the Torah states: “Neither let it be heard out of your mouth,” which includes causing a gentile to take an oath in the name of an idol."
What about Xmas ornaments that say "Happy Holidays"? - See [Gittin on Shulchan Arukh, Yoreh De'ah 148.9]
"The mishna teaches that one may extend greetings to gentiles on account of the ways of peace. The Gemara asks: Now that it is taught that one may assist them, is it necessary to say that one may extend greetings to them - Rav Yeiva said: This halakha is necessary only on their holidays, as it is taught in a baraita: A person may not enter the home of a gentile on his holiday and extend greetings to him, as it appears that he is blessing him in honor of his holiday. If he encounters him in the market, he may greet him in an undertone and in a solemn manner, so that he does not appear to be rejoicing with him."