I believe it is unfair for you to criticize my colleague when he is asking all these things of you for a reason. We are ALL licensed attorneys here and we have all had education/experience with regard to defamation cases. Without going into all the details, there was a reason Mr. Holloman was asking you those questions--even if it might not be apparent to a lay person.
That being said, what you potentially have going on here is libel, not slander. Libel is the written form of defamation (slander is the spoken form). California Civil Code section 45 provides in part as follows:
45. Libel is a false and unprivileged publication by writing, printing, picture, effigy, or other fixed representation to the eye, which exposes any person to hatred, contempt, ridicule, or obloquy, or which causes him to be shunned or avoided, or which has a tendency to injure him in his occupation.
The Internet has made the ability to speak ill of another person online much easier and as a result, claims of defamation have the potential to grow commensurately. The above parameters of what constitutes defamation also apply online, subject to the provisions of 47 USC section 230. For a fuller discussion of issues related to online defamation, read this excellent article here: http://www.ivanhoffman.com/defamationinternet.html
Basically, CA has followed the majority of other states by holding that the actual publisher (Craigslist) of the defamatory content cannot be held liable, but the person who writes the content can.
If what this person wrote is potentially damaging to your business reputation (and it sounds like it is), then you may want to bring suit. Normally, I would suggest that you first have an attorney issue a cease and desist letter on your behalf, but the problem is that this guy (or girl) is publishing these statements anonymously. As such, you are going to have to sue "John or Jane Doe" and then subpoena the IP address of this person from Craigslist.com. After a little more detective work by your attorney, you will probably get this person's name and then can go after him directly. It will be costly, and in the end, you will have to prove that you have damages from this defamation. Regardless, this is the course of action you must take if you want to sue this person. If you have more questions, I will be happy to answer them. Please accept my answer so I can receive credit for helping you. We can always continue our conversation at no additional charge. Thanks!