Any variant of my answer will boil down to being built around "well... people".
Maybe ease of use is the weak point - I personally doubt it - go for fixing it.
At the same time, I do suggest that there is something more complex bubbling under the surface.
GO for building your alternative. Nothing works better to figure out whats going on than an example that can be tested against. Good luck! Interface isn't everything!
Either improve craigslist or let others build value added tools on top of craigslist. Why stop an app like Bazaar that helped people post things easily? Let people make up their own mind. Using smartphones as an example again, people will trade int their iPhone 3GS for $80, when they can get $160 for it on CL. So there is definitely demand somewhere in the massive gap of services that retailers and B2C companies offer and what exists on craigslist. Making consumer to consumer commerce smoother and easier is a huge opportunity in my mind that is not only lucrative, but fills in a big gap that exists because craigslist is still stuck in 1995.