You could also try a credit based system. Every time someone gives away a cigarette (which I would always do when I smoked) they earn a credit that allows them to bum one from other participants in the future.

You could let people go "negative credit" too, and let other smokers choose a threshold below which they won't accept requests from moochers. "Man if someone is at -10 he should just buy a pack and give something back"

Then, when you are "burdened" with a pack, you can give them away, since you were probably going to throw it out anyway, and happily bum-a-smoke 18 times in the future before buying a new pack.

This avoids many of the legal and payment processing problems you might run in to if money is changing hands.