This due in part to the way images get sold (one image, but multiple image sizes and pricing). Not only that, but I don't do any credit card processing - that all gets done by my supplier. To make matters worse - I am not the one selling stuff - the users are! I have to keep track of who is selling what, for how much, and how much money do I owe them (they get around 50% of the sales price).

That being said, I don't see much work to write a generic bare bones shopping cart application. Things will get tricky when you have multiple sellers.