I buy. . . nada. First, those snacks are waaaaaaay over-priced. I'm a cheap bastard, and the tickets alone are expensive enough as it is.
Secondly, I get annoyed with all of the rude drink-slurpers and popcorn-smackers making all kinds of noise during the flick, so I'd be a hypocrite if I were to do it myself.
Plus, I don't want anything to get in the way of my enjoyment of or concentration on the movie itself.
Yeah, I'm a real fun guy.