Yes: buying stuff from people is pretty much instrumentalising them. That's capitalism! Although there tend to be limits as you note. And the 'would they like this if they knew what I was doing' is obviously a very good rule of thumb.

Occasionally, you'll have to break this. Sometimes somebody is irrationally self-destructive and you basically end up deciding that you have a better sense of what is best for them. But that's an INCREDIBLY radical/bold decision to make and shouldn't be done lightly.