I'd skip the xt in favor of the xti. It was a big improvement
Disagree! I for one think the XT is better, mostly because I just love having the monochrome LCD screen (even if it is on the back...).
I wouldn't shop for an expensive digital camera on craigslist...but if you do, bring along a laptop and a CF card so you can check the pictures to make sure you don't see anything wrong (scratches on the sensor's filter, lines indicative of a failing shutter, etc.) and so that you can check to see that it shoots, autofocuses, etc.
Basically make the camera run through everything you would be using it for. If there's anything odd about it, then don't buy it.