Check the question that Jim Cavoto asked about why his hundreds of Craigslist ads were being flagged, and see his responses. Craigslist is very clear in their guidelines about posting ONE ad, once a day, posting locally for ONE thing, etc. to avoid realtors who are spammers. It is very tempting I think for realtors to use this service (and cheater software to bypass the regulations if they are truly dishonest) and save more of their commission for themselves instead of investing it in the actual marketing they are hired to do. Craigslist is very clear it is meant to be a small-town, grass-roots, local private classifieds run by one man as a free service, from one person to the next, not a commercial free-for-all.
That's all there is to it.