Why not let him get his 4 post in per month, then stop them from there on out. What he does on craigslist is his own business not ours, and our forums shouldn't be saying "you use CL to so that counts against your post here."
My 02 cents. I don't care if he sells a million fish out of his house, if he stays within the rules of 4 FS post then let it be. The sponsors of this forum pay to get their own section and to not compete against others in a general for sale sections. It's like having a store front vs selling at a flea market. It's all about exposure and you already limit that by the number of FS threads he's aloud to have/location of post.
Just to be clear, I don't think people should run a business out of their house like this unless prepared, and if it's a true business then he needs to pay the expenses one takes on by having the business. My post is more about fair enforcement of the rules, which it seems everyone bitches about here in this forum.