Maybe it's because I see the question as a bit condescending, or I just don't like being probed. But my standard response to that kind of question is similar: "whatever I want." Now the real answer is a target, and maybe varmints. Genuine interest gets a genuine response. Condescending interrogatives are meant with ambiguous sarcasm.
I disagree that that the clerk asking what he was trying to shoot at 500 yards was anything other than an inquisitive response.
The clerk may have simply been trying to find out what kind of target so he could recommend the most appropriate gun for the customer's needs.
Look at this situation objectively:
The man walked in and asked the clerk "What gun do you have that will shoot 500 yards?" This is fairly uneducated firearms question with a very specific target distance.
Anyone who is actually capable of hitting targets at 500yds would already know what guns are capable of shooting 500yds, and probably wouldn't ask that question.
All the clerk really did was offer the man an opportunity to back out of making a obviously uneducated and seemingly spontaneous purchase.
The man wasn't refused a sale, or treated badly by the clerk.
Perhaps the clerk enlightened him to the fact that maybe he isn't experienced enough to be attempting to shoot 500yds, and an AR-15 wasn't really something he needed.