Right above the bold black text that identifies the item being sold there are:
a box with reply, then the words favorite, hide, flag.
By clicking on the flag word it shows that you beleive that particular listing violates one or more of Craigslists' rules.
Or, from the official CL page,
Flags and Community Moderation
craigslist users self-publish tens of millions of free postings each month, subject to the CL
terms of use (TOU).
CL users flag
prohibited postings they find via the flagging link at the top of each posting.
Free classified ads sufficiently flagged are subject to automated removal.
Postings may also be flagged for removal by CL staff or CL automated systems.
Millions of ads are removed by flagging monthly, nearly all of which violate the CL
terms of use.
Of course no moderation system is perfect, and a small percentage of ads removed are compliant.
Flagged postings that comply with the
TOU may be reposted, reworded as necessary.
For help from CL users regarding flagging, visit
flag help forum, and follow the instructions there.
Funny or memorable postings may be nominated for "
best of craigslist" via the "best-of" link.