>In reality, they're just trying to sell their stuff, I don't understand why someone would react by yelling and getting angry.

In that case what you're doing seems to be significantly worse than saying "fuck off".

Maliciously wasting their time vs making it clear you aren't interested?

FWIW, I don't think there's anything wrong with what you're doing. I just think it's objectively worse than "yelling and getting angry".