So there are a few things you’ll want to take a look at… And let me clarify the behavior you’re seeing too…
First, are you saying that when you actually make code changes to the wpsc-default.css file and save them, when you inspect the code after you refresh the page, your changes are not there? If that’s the case, it sounds like you could either have a caching plugin installed or your host might be caching files for you. (The first scenario is more likely). If you have a caching plugin, you’ll want to temporarily disable it during development.
If that’s not the case, and you can successfully make changes to the code and you can see those changes in the code when you refresh your browser, I’d recommend actually making your changes in style.css which is located under your theme folder. It looks like that file is being loaded after wpsc-default.css, which means you can keep wpsc-default.css pristine (in the event you can upgrade it some day) and your overrides are in your actual theme code.
All of that said, this is probably more of a CSS/HTML question (than a WordPress question). There are any number of ways you can adjust the layout to do what you want. Just as a quick example, you can add these styles to style.css and it should get you on the right track.
.wpsc_categories {
float: right;
}
.wpsc_default_product_list {
float:left;
}
Of course, that might impact how the WP E-Commerce framework styles things by default and such. I don’t know anything about WP E-Commerce.