does 280x perform better? it is a rebrand but I thought there was a performance increase? and it is also 120$ for second hands.
I seen on ebay some 7950s 100-130 bucks, overclock it and get close to the perfomance of 7970ghz.
One thing I would keep in mind is that many of the 7950 and 7970's have been used for bitcoin mining and may have been a bit abused. They were being sold at the peak of the craze. One of my acquaintances, for example is looking to get rid of about 5 of them. As you mention though, if you are okay with the risk used 7970s are pretty good bang for the buck.
Sometimes they are less abused than ones that weren't mined on though.