I've used Cart66 (used to be called phpurchase) and I got it to work well for me after a little bit of hacking, although that was because I wanted what amounted to several different products on a single "order form" for event tickets. It does the job though and there's a free version available. The main difference with it is that it really only implements the product/inventory management and checkout/payment functions. How you define and structure the products is then entirely up to you - good for small numbers of products, but could be a lot of work for lots of them.
The premium version includes subscriptions and subscription-only content on your blog, which in some cases could be useful, depending on what you're using it for.
Paul
Paul I evaluated cart66 but found it quite intense, I really feel non-tech users will find this rather frightning and cumbersome, the logic just doesnt flow IMO