Answer

In my opinion there is no prohibition if we are speaking about atheists (more precisely, people who do not accept halakhic obligation from Sinai). Not every secular person is an atheist. There are also people who are just careless about these things.
Buying Sabbatical-year produce is problematic, since you are taking an active part in the commerce and not merely causing him to stumble. It is somewhat like directly feeding something forbidden by hand, which is prohibited even in the case of a minor (Yevamot 113-114).