For a theme, use any of the blank themes out there [Foundation, Bootsnap, Pure, UI-Kit, Foundry or Free-Stacks) and make the design how you like it.
For the selling, there are stacks like RapidCart, Payloom or Paysnap (others too, one by Yuzool I think). There are also cart systems out there, Cartloom and Ecwid.
If you want something wordpress like, take a look at the blog function of Total CMS. People have easily integrated Cartloom into it I know. It has a cards blog type thing you can see here:
As far as something like Craigslist, only one thing, Mootable stack by Stacks4Stacks