I for one think that what you're doing is just fine. It's your weapon, and you are free to choose whom you sell to. You're not taking away anyone's 2A rights; you're not saying "thou shalt not have a firearm," just "thou shalt not have my firearm." We often are forced to make character judgments, whether we will trust someone we don't know, and you should feel fully justified in your decision not to sell to someone that you don't trust.Bear in mind that I did mention in #2 that appearances can be decieving. The sales I have refused were to individuals who made me uncomfortable. I've seen and known many "unsavory characters" in 3 piece suits.
I am in no way implying that it is ok to be prejudicial, however, it's hard for me to believe that no one has denied a sale because of political correctness.
Now, if I was the one selling the weapon, I would never make a blanket rule like "no selling to Arabs." But that doesn't seem to be what you're doing.