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.
A shop is a great system on a forum as it gives the members something to save enough 'credits' for. A lot of forums have a points/credits system which can they be used to purchase account upgrades such as extra private message space, a custom rank or even access to private areas. If you are looking to add a little fun to the shop, you could always add a lottery, which members usually enjoy taking part in.
I've just got a website already built with a shop, as a present. It has a lot of posts on it and a decent FB page with some 6K likes. I'm wondering is it good or bad, having a shop on it. I'm not sure how to handle it right away because it doesn't have anything with what I do.