I use my own child theme based on 2010 and just wp ecommerce plugin. It works great, I have a magazine/news type web site with lots of esction and just sell stuff as an affiliate through the shop section.
You do need to be a bit careful with some specialist retail themes as they can make it a lot harder to customise things the way that you want. I've done a lot of hacks and mods with wp-ecommerce 3.8 that work great with basic themes but will not work with specialist wp-ecommerce themes as they use a lot of their own coding.
The other thing to remember is that the more basic and nearer to the default 2010 or 2011 theme you use the less likihood there is of you running into conflicts with plugins. Most developers will test their plugins against those themes and not others.