I got mine on sale for ~240$ and while I love my phone, it might be worth going for a newer phone. But if you really want the triumph, then here's my advice:
If it's on craigslist, I would recommend asking them to test it out first. Make a test call, send a couple of test texts, try out the android market and whatnot. You know what you use frequently in android and you need to be sure it works(also, try meeting them in a coffeeshop so you can test wifi).
Sure you can't guarantee a good battery life by just playing with it a little, but you can order a new one for cheap off amazon(I paid about 5$ for a "low quality" Chinese extended battery which lasts as long as the stock one does out of the box, it'll take a couple weeks to get to ya tho.
If it's not activated, you want to try and check to ensure the phone hasn't been stolen.
Also, I'd recommend really only buying it if it's un-rooted, and rooting it yourself. Something just tells me that buying a rooted used phone is a bad idea in terms of safety/personal data. *shrug*. I guess if it IS rooted, you can just flash a fresh ROM to it tho. *shrug*