Even a great idea is worth nothing compared with the work of implementing it. And my guess would be that the ideas you'd find on craigslist are more along the lines of "build a twitter clone only MORE SO" than any genuine insights.
Even if you do think the idea is amazing, only engage in such a deal if you get to keep 90% of profits. Development work requires hundreds of man hours. Ideas can come while in the shower.
It's the difference between "Dude, I should learn to play the guitar" and practicing an hour a day for five years.
