Hi Malaika, I found my first client (she's still with us!) on Craigslist years ago, so it can be a source of potential clients to be sure. There's also a lot of spam, junk and people looking for the lowest possible rate so while it won't cost you anything to take the time to respond, and you should if you're interested in the client, just do so knowing that it may not pan out.
There are quite a few conversations regarding responding to classifieds on Craigslist, etc. in the
Marketing section that you might find interesting.
If you are going to reply, I would just treat it like any other RFP - respond as thoroughly and professionally as possible. Let them know you have the solution to their problems, etc.
Good luck!