In most cases, considering how long it would take to set up a site like that, you wouldn't even be getting close to minimum wage.
Hypothetical situation: Install something like Zencart or Drupal+Ubercart, use a pre-made theme, out-of-box configuration, and not teaching the client how to use it at all, you could have such a site up in a handful of hours. Maybe. But what kind of solution would that even be for the client?
Lurking around those freelance job auction sites is what inspired me to post this. Not only is it impossible to ever get work on them, which SO many people outbidding one another, but the winners always end up working for lint. It's shocking.
I wouldn't do it. It's completely undervaluing yourself to work for scratch like that. I mean if you were a student and it was work experience for getting your protfolio up, then fine...But you can't run a business with costs that low.