Craiglist a "simple website"? LOL You do not even know the back-end, the servers behind such a massive site. You have no clue about it. A site that gets that amount of hits is not only about the Front-End, more important is the Back-End. Okay, then show us your "not simple websites". Are they getting more traffic than Craiglist? Example: Google, looks simple. Simple website, done in 2 hours (with their CSS sprite). This site is not simple, it's fast. And it focuses on ... searching. The only thing this website is made for. --- You tell him you would do the site yourself. He has no knowledge. Can you code a site with webshop? Do you know how much time it takes to develop a webshop? For a one-man show... 12+ months? If he has to learn HTML/CSS from scratch there is no way to do a kick-ass website with shop in 12 months.
Useless post from you. He does not want to have a website, he wants an eCommerce solution. Read the post before wasting time.