This is coming from personal experience so take it with a grain of salt:

Craigslist is an insanely valuable source for lead generation in recruiting, and car sales.

I run a recruiting company and craigslist is the best source of applicants by volume and return on investment (cost ($) per applicant).

Having observed them closely, they are not ignorant to this fact.

When specific cities or sections grow more popular they begin charging fees.

Their biggest boon has been a recent change to charge car dealerships 5$ per post. Car dealers pay because only months earlier they were paying for black hat solutions to post for 1-2$ per post.