With my forum which is a general discussion forum I have a shop which is in the form of a thread which sells things like account changes (username changes, usertitle changes), personalisations (colour changes), upgrades and the ability to buy a custom usergroup. All these items can be bought via our forum currency (credits) or the expensive ones can also be bought via paypal. I think it's good to have a shop as it acts as a reward for user's posts.