Selling is not convincing, selling is finding people that are uninformed but are in the market for your product. So good sales people will stop selling to you when they realize you are currently not in the market for the product and move on. Bad sales people will waste their time "convincing" you.
I have a co-worker that I regularly talk to about stuff that he has some interest in, like games. Quite often, he'll be uninterested in a certain game, until I start talking about it. Afterwards, he's very interested in it. He has repeatedly remarked on this situation and how it affects him.
That leaves me with a moral dilemma: Am I influencing him too much? My goal has never been to get him to buy something. I've only ever talked about my interest in it and why I want it. But I feel bad whenever he ends up spending money based on my interests when he already has his own.
"You aren't allowed to persuade anybody of anything".
It's great advice because you can't really convince people of things anyway unless you have a lot of time or a lot of money.
When you don't have to worry about changing people's minds selling becomes a lot easier.