The existing user base of CL, and the chicken-and-egg problem of buyers and sellers, are definitely valid issues.
However I would keep a very simple interface. There are things that I have sold on craigslist and I knew that I would get more and sell it faster on ebay, but right then I just wanted to get it "in the process" of getting gone, and ebay is a pain. It may be because of the computer or browser I use, but it is slow, it loads up all kinds of ads and kruft to show me what I want in a tiny 2 square inches in the center of my big monitor.
In fact, I would go further, and provide RSS feed interfaces and even a newsgroup interface ( not many people will use that, so maybe make a few million first).