Ah craigslist, how I love thee.
First off price. Get exactly what he's going to be selling to you and compare it to what the same gear would cost you new. If he's asking more than 50% retail he's off his rocker. If he's asking 30% or less give it some serious thought. Remember, you're buying used and have no idea what he's REALLY done with it.
The filter will have to be restarted, no getting around it. Once the filter dries out the beneficial bacteria die and that's the end of it, now its just plastic and a cool paddle-wheel. Read up on cycling as you're going to have to do it. http://www.fishforum.com/freshwater-...ium-cycle-252/
I can give more detailed advice on cycling if you want it but my biggest thing, Do not trust the bacteria in a bottle. I speak from experience.
The ich treatment should raise flags if you're planning on keeping invertebrates. Shrimp, snails, corals if you do saltwater, all are highly intolerant of copper. Most ich treatments contain copper, lots of it, because the ich parasites are invertebrates. At this point things are hit and miss. Depending on how often and how severly the tank was treated some of the copper may have been left behind. There's really no way you're going to be able to tell without buying a copper test kit and testing it after you fill the tank. Alternately, and I'm sure this will draw some stern glares from some quarters, you can always toss in a few ghost shrimp. Its up to you, twenty bucks for some copper tests you'll use once or a dollar on a half dozen feeder ghost shrimp, your choice. It sounds a bit heartless but that's how I tested my tank, tossed in an amano shrimp and hoped for the best. Yes, he's still going strong as are his two buddies. Of course if you don't care about keeping invertebrates you're in the clear.