I’m pretty sure the answer is “it depends”. Specifically, it depends on whether the vendor knows the intended use of the item. If you know or have reason to suspect that a person is going to commit a murder, then selling them a gun makes you an accessory before the fact to the murder.
Similarly, if Chester Christen Baker bakes a heterosexual wedding cake, and then somebody switches the bride figurine to a groom figurine, he hasn’t violated his conscience. But if he is asked to decorate a cake and put two groom figurines on it then it is possible that it would violate his conscience.