The backend doesn't seem that simple to me. Maybe it's just me.
It's written in a standard Perl LAMP stack, and to my knowledge hasn't changed much since inception. It's not as simple as a converter script, sure, but by today's standards it's extremely simple.
How does craigslist make money? Ads?
AFAIK they charge money for some ads, such as job postings and housing ads.
I believe they only charge to post job ads in big cities like New York and San Fransisco.