I don't think she was selling it on Craigslist. She just sent an e-mail out to a few of us saying to let her know if we knew anyone who'd want to run it.
I know that they don't like people running bandit (like the guy at the NYC half who took his shirt w/bib off mid-race and was detained before the finish line) but who knew about running w/other people's bibs!?
It's listed in their Rules of Competition on the website. But they have employees who search for the CL & other ads for bibs for sale, then try to set up a sting to catch you when you pick up the bib. There are a lot of posts on the Runners World forums about it.