DM, but correlated (not causation).
I spend a lot on miniatures, and I pick up books when they look interesting. One friends of mine picks up books. The others play.
I'm GMing a campaign, and the book friend is GMing a campaign. If we were players, we'd still be spending the money. We both like having new material, and we like things to be _ours_. The other gamers have other things to spend money on.