A good example of an open source ERP is webERP which I have set up here. Feel free to poke around to see what it does. It's very quick to install and can be used in many instances with virtually no modification.
Putting up a web store is also easy now with the relevant drupal module (ubercart) or good software like oscommerce available open source. Have a look at the ubercart store and oscommerce store set up on this site. (you can play around but could end up actually buying some software!)