Well, you should exercise the same care you would in buying anything on Craigslist or anything similar. Certainly, you should look it over and play the instrument. Ideally, I think you might want to have someone like a piano tuner, or someone (a professional piano tuner could probably give you a pretty good overview of the piano’s state/status), look it over before you buy it, like you might have a mechanic look over a used car before you buy it. You are going to pay a certain amount for the instrument, you will probably pay to have it transported to your home, and, if it turns out to be a piece of crap, you will have to pay someone to remove it from your home. A piano is a pretty big thing and people don’t haul them around for nothing. It might be worth it to have a professional look the thing over before you buy it; that’s pretty much the gist of it.