Help with craigslist purchase needed!

Hello guys/gals I have a problem. So I'm in the market for a 15" mbp and I stumbled upon one on craigslist. A brand new unopend 2.66 ghz mbp for $1700. I thought for sure it was a scam but thought I might as well email the guy about it. So far everything checks out the serial # hasn't been registered, apple verified the purchase date which was the 16th and the guy apparantly got it as some sort of gift. The person who gave it to them bought it from best buy. So still not knowing for sure if this was legit or not I asked the seller to go to best buy and get a copy of the receipt and he said he did, but apparantly they bought a bunch of other stuff as well. So that kind of raised a red flag as to whether or not they used a stolen credit card. Also the seller said he would lower the price to $1600 so that's $400 off... Why would they sell it for so cheap?! What do you guys think? Should I buy it? If I do and it turns out it was stolen will I be held responsible even though I did everything I could to find out if it had been stolen? Argh I don't know what to do please help!