Wait until they repost it for the 6th time within a month, then lowball.

In all seriousness, I do see people with unrealistic expectations on craigslist, but the idiots doing that don't sell their bikes, and you do see them reposting them time and again. A lot of people settle for between 40-70% of retail - depending on age, mileage, and desirability of the bike. Higher end bikes retain better value. Guys with wives who want their basement back will generally take what they can get.

I got a bike off Craigslist (Giant 2003 OCR2) that originally would have sold for about $800, the original owner got it for $700 in an end-of-season clearance, and I got it 3 years and maybe 50 miles later for $400. I probably could have haggled, but it was a good deal and he threw in the shoes (which happened to be exactly my size). I'm not kidding - this guy couldn't find the pump he bought but apparantly never used, and took 10 minutes fiddling with a neighbor's pump before he figured out the Presta valves.