Society affects people more broadly than just whether they can buy a new car that day. The "libertarian" way is a larger impact than just you and one customer. If you don't sell that car, and your neighbor doesn't either, and you cause a general neighborhood tendency toward not selling cars to black people, then that person may have trouble buying a car anywhere, which may make it hard to drive to a place where he can work. And if landlords behave the same way, he may have trouble finding a place to live, and overall it becomes more problematic for him to live at all in the country where he was born, which will make him ever more undesirable to people who are making superficial judgments.

I mean, we've been through this, and I really think you should read some history. Nothing is just "libertarian". We are affected by the choices around us, and those choices affect other people exponentially. Society is a web of interlocked actions. You can say you have a "right" to do whatever thing you are interested in doing, but it is also important to look at the bigger picture, at how it will impact the social structure, at how it would affect you to be told that certain things are now against a law. Things like wearing a seatbelt or drinking under age or while driving, or smoking in hospitals or on planes were legal until laws were passed. These things are your choices, but will make a difference to the whole world.