Well, of course you can reproduce their design. But reproducing their design won't reproduce their success. If it were that simple, every website would look like Craigslist.

Again: you're missing the point. Craigslist is the exception to the rule. It's a product of its time. For any value of X, if you tried to say "this is the way Craigslist did X, therefore I should too," you'd very likely be wrong. And in any case, Craiglist is pretty far from the romantic notion of three guys in a garage. It's an expensive site to run, even if it's more cheaply run than other big sites.