I use ecommerce wizard. Their support is really helpful when you're getting started. Also, you don't need to know about the whole html php mysql thing! Once I registered, they setup an online shop for me.
Instead of using open source software, they stress that your shop runs on commercial software - and I have to say the feature list is extensive - but don't let that put you off. You get to choose a template from a large selection (which are very, very, very good by the way!). But I guess most people want to change the design of their shop.
Although I havn't used it much (havn't seen the need), there is an online WYSIWYG editor so you can easily change the apearance of your template. But I choose them because of the price tag which will cost you £45.00 per year.
So far, I'm really happy and have been getting great results. However, there are others (e.g. ekmpowershop). They are usually more expensive and harder to use. You also don't get a great design unless you pay them to design your shop for you - or have all the time in the world to learn the ins and outs of website design.
I suggest that you give
ecommerce wizard a look though...