"Professional" and "Craigslist" don't normally go together.
I mostly agree, though I do see some reputable big companies use it. However, the majority of creative gigs on Craigslist are low paying, targeting the inexperienced and uninformed, and not worth your time in my humble opinion. Some savvy folk find good jobs, but sorting through all the bad ones makes me tired and jaded.
To do a broader search of Craigslist, Google something like: "writer wanted" site:craigslist.org
As for creating a professional post, there's a lot of online guides and "how to" sites, but there's a two things that'd make me immediately dismiss any "writer for hire" ad:
1) Typos and/or bad grammar
2) Vague or unnamed references and/or awards