Sorry to hear it, but it certainly is a bit risky to buy a 3-year old used console from someone you don't know; who knows how they treated the system. The yellow light just means general error, it could be all sorts of things; broken hdd, broken psu or bad connection of some of the chips (which can be solved by applying new heat paste and reflowing the cpu chips with a heat gun, search youtube for a guide).
"Well certainly with the Xbox 360, we had some challenges at the launch. Once we identified that we took control of it. We wanted to do it right by our customers. Our customers are very important to us." -Larry "Major Nelson" Hryb (10/2013). Note: RRoD was fixed with the Jasper-revision 3 years after the launch of 360
"People don't pay attention to a lot of the details."-Yusuf Mehdi explaining why Xbone DRM scheme would succeed
"Fortunately we have a product for people who aren't able to get some form of connectivity; it's called Xbox 360,”-Don Mattrick
"The region locking of the 3DS wasn't done for profits on games"-MDMAlliance
