Not sure where you stand on this now, but the 5500 should be a great deal at that price as long as the bike and components are in good condition. A 5500 is a pro-level bike and if you can get it in good condition at that price, it's a great find!
I assume it's in your size, right??? Also, I don't know anything about craigslist, so I don't know how often people get jipped on there. Are you able to see the bike before you purchase or is it going to be shipped to you? Try to make sure that it hasn't been wrecked.
Be careful with all that stuff, but as a bike, the 5500 is great. I've riden the 5500, 5200, and 5000 from Trek and they are all great rides. With the aerobars and the seat all the way forward (I actually use a Thomson "set-back" seatpost flipped around to get even farther forward) it's a great tri bike!