Yes, it does seem to depend on how one gets to a person's list. This is what one sees if going from the Plant List link in the left navigation bar:
http://garden.org/lists/
All it says at the top is "Plant inventory, trading and selling lists", followed by a list of users with nothing to indicate up front who is willing to sell or trade, and it isn't necessarily clear when clicking on a person's list either.
However, I did discover after my previous post that if one goes a different route to peoples' lists, and now I can't remember how I did it of course, one does get the notice about not bugging people for plants.
