I also agree with Basil in as much as Zarath's shop mod works, but is pretty cryptic witha large learning curve making it difficult to master. I spent many hours redoing it for my needs on phpBB, but once I finished that, coupled with Cash Mod and Vault... It made an awesome rpg setup. I think the same thing is achievable with SMF Shop. IPB Shop is a real sweet piece of work to be sure... but in it's diaper daze, it started just like SMF Shop. It pioneered greater functionality by taking advantage of the fancy bells and whistles Microsoft wanted to corner the market with in IExplorer. I truly believe that everything IPB Shop might be today, SMF Shop can just as easily become. The architecture of it allows for just about endless Items to be added... ALL those things need, is just a creator/author.
ta-DAAA my soapbox for today! :p