I've never had any dealing with craigslist, being an aussie, but i've seen a few on ebay (which does pride itself on having cleaned up a lot of scams compared to when it started).
i saw an EF 14/2.8 going for cheap, starting at $1 auction, and put it on my watchlist. I would have bid on it, but the seller was in Thailand, with a 0 feedback rating (auction was in aussie dollars, and listed on ebay.com.au). It finished around $1400 (c.f. B&H new price $2300), and I kept it in my watch list. Whoever the buyer was, they never left positive feedback to the seller, not sure if they received it or not. Either way, after a few months, the seller was listed as 'no longer a registered user'. I know ebay say you're protected, but generally only $400, and with fees and whatnot you never get back the full amount.
Same thing recently, a Zeiss T* 35/2 went for $757, to a seller from china with 0 rating. I was planning to watch the item again for a few months to see if it had a happy ending, but it seems not already, the seller is -1 feedback with the comment "This seller sold the item through another netshop the day after I won the bid". So at the least it looks like this buyer didn't lose any money.
It's a bit similar to the craglist scam though, could have easily turned into a "but i've got another lens i can sell you...".
long story short, always check feedback from sellers, especially if you're handing over a lot of cash (only pay $1-2 max for every positive feedback they've got is my rule).
and if it's too good to be true, it normally is...