About three days ago I finally got my hands on a legit, brand new in the box, 3GS 16GB. Paid $500, without even trying to negotiate.
I've been looking for a replacement for my 2G, with a broken screen, for about a month. Most of the Craigslist ads are bogus, asking you for a phone number and you never hear back. The worst ones are when you set a place and time to meet, and they do not show up, or come up with a sorry excuse.
For one offer, I even made a 60 mile roundtrip, and the guy never shoved up. Because of his unique name and smaller town he lived in, I white-paged him, found his phone number and gave his mom a call. She profusely apologized about it, and told me she'll have him call me. Never heard anything back.
eBay was a no-go as well, since all of the ones I was interested in went way above $500, used! For the unlocked ones it was even higher, in some cases $1200. Way to much.