I can understand OP's conflict, if the word was said without rancor and not preceded by the words "dirty" "lazy" or "g-damn" which would clearly convey hostility toward blacks. I personally think too much is made about the N word when it is not said with hostility. It's just a friggin word. It used to simply be the word used to refer to blacks -- neutrally, without hostility, it was just the word used. When Huck Finn referred to his friend "Ni**er Jim" he did not mean anything derogatory by it. Somewhere in the 60's the word became a pejorative, not just a reference to people of dark skin. And now we have this odd situations where blacks can use the word, but whites can't.
Curious -- was the salesman black?
Some people grow up in an environment where inappropriate words, racial slurs, and F-bombs are common. They get used to hearing them, and even using them. Even though they are not appropriate in the workplace, they can slip out accidentally. However, if that is what occurred, the salesman should have immediately been flustered and said, "I'm sorry, I didn't mean to use that word." So I'm not sure how to call this one.
I agree it needs to be mentioned. He should not use that word in the workplace, whether he is white or black and whether the customer is white or black. It should not be used, because many people find it offensive and we should not be offending our customers, whether deliberately or inadvertently.
But I disagree with the statement that a person cannot be nice and friendly and still let that word slip out. Certainly people can be nice and friendly and still commit social faux pas.
I have a client from New York who refers to gays as "fags" which jolts me because I have several gays and lesbians in my acquaintance (and have gay characters in my books) and around here, calling someone a fag is not a term of endearment, but I have not judged him on it because I have not felt disgust from him when he used the word and I suspect this may be a regional thing. Not living in New York myself, I don't know if it is common to use that word instead of "gay" or "queer." So until I know more, I am reluctant to judge him for using a word that may be nothing more than the commonly used term where he came from. If I ever sense an attitude from him about it, I will call him down on the attitude. But I'm not going to judge him or presume him to be homophobic because of the word he used.
I'm more concerned with hostile attitudes and discrimination than with the word used to refer to someone who is not actually present to be offended by it. I don't need to be offended on someone else's behalf. I think blacks probably have less issue with being referred to as "ni**ers" than they would with being referred to as "f-ing African-Americans."
But if someone said to me, "I can't stand the fact that so many f-ing blacks have moved into my neighborhood" then I will judge that person racist and not nice and not associate with that person -- even though they didn't use the N word. Someone who has many black friends and hangs out with them and hears them call each other the n word all the time might easily accidentally refer to his friends as "my ni**er buddies" without thinking about it.
Judge the attitude, not the word.
But report the incident so the salesman can be corrected.
Time to build a bigger bridge.