The second comment on this thread that suggests that the problem is "building an open replacement". Have you been on Craiglist lately?

You could try to replace Craigslist using only variants of the code shown in the original Rails screencast, with no consultation allowed. But you'd fail, because the result would be (from a UX perspective) better than Craigslist.

When people find a forum in which to post classifieds, they have exactly one (1) overriding goal: find the forum with the most vibrant market. There are a million zillion little things Craigslist can do better, but they are unlikely to make a big impact on that (1) goal, because Craigslist is already so vibrant that it has mostly put the rest of the classifieds "industry" (ie: newspapers) out of business.