When you just wave the off and walk past you never enter the danger zone. Things get increasingly more difficult once you engage in conversation with them as they won't let you go unless you rudely quit and walk away, and most people don't want to do that. The logic goes that since you've already spent a while talking with them, spending your time and their time, why stop so suddenly and throw all that away unless you end up buying something to warrant the effort made. It's basically bullshit but that's how many people go about it: you either decline right from the start or end up "having to" buy something. It takes some strenght to break out of that pattern.