I thought this was a joke thread, but I guess this scam is new to some. This scam attempt is replied to a vast majority of all Craigslist for sale postings and has been going on for the last decade. Its mostly Nigerians and always involves them wanting to buy whatever your selling on Craigslist and immediately sending a bogus check for more than is needed with the idea that you'll quickly mail them back the difference in a legit check.
You can have some fun with them and make their life a little miserable, here are some tips:
http://www.419eater.com/html/baiting.htm