The problem with attacking that weak point is that the amount of annoying trash on craigslist is apparently dwarfed by the useful non-trash, despite the possibility that other sites may a lower actual percentage of trash. Which is most likely a function of popularity: a just-as-popular competitor would have just as much trash as craigslist. But competition is not just about user experience, if you had a way to improve the signal-to-noise ratio, you'd have to do it for cheaper than craigslist does it, and considering the amount of trash, they aren't spending a whole lot of money on fixing this weak point.

I mean, you spelled it out right there: if you don't find what you want on craigslist _first_, then you go someplace else. craigslist apparently has enough value, and it's easy enough to determine if you're going to get any value out of any one visit, that you don't bother going someplace else _first_. It is not a last resort visit, the craiglist competitor sites are the last resorts.

So it's the same answer again: craigslist is where majority of the users are.