You're providing no evidence to suggest that racism is an issue here. Sure, it could have been. It could also have been that he didn't like your shirt. Or that he mistakenly thought you were going to try to use an enormous jar of Canadian pennies to pay for the TV. Or that you looked like someone his boyfriend used to date. Or that he was planning to murder you later on and didn't want to arouse suspicion by being seen with you on a surveillance video. Or that he was a time traveler and, having recognized you as his own father, was worried about whether selling you a TV could result in his never being born at all.

Racism is really awful. So is being falsely accused of racism. If you're going to suggest that something ugly was involved, you kind of need to have some kind of evidence besides, "Well, he and the guy he was helping were from one race, and I'm from another." If you *have* better proof -- which I doubt, because it didn't occur to you until you talked to your friend -- then I'd suggest you rewrite this letter to make it clear.

Yes, he should have acknowledged you, given you a chance to say, "Hey, I'd like some help," and either helped you promptly or said, "I'm sorry, that man has been wandering around here for awhile but I'll be with you as soon as I can" or something. He didn't do those things. And that's a valid complaint, but it's not a significant (let alone horrible) enough valid complaint that the whole "My friend said it might be racism and now I want you to have diversity training" doesn't completely overwhelm it.

Also, if you want your "You lost more than $3,000 because of what may have possibly been racism according to my friend" to have any punch, you may want to revise "I am seriously considering spending my more than $3,000 at a store other than yours even though they don't have what I want" to something stronger.