Yes, it's a big upgrade. But the fact that is a big upgrade, what will it matter if it's a faulty GPU or if it fails shortly after you have it in your system?
A GPU used for mining is pushed to the limit for who knows how many months (possibly years, but let's say months). It's far easier to fail than a GPU used for gaming and most of the time they run with edited vBIOS (mining) that may have even bricked the card in the end.
At that price you either found a VERY good deal from someone who doesn't know about the current GPU market, or a mining GPU that the seller is trying to get rid off for who knows what reason (faulty, bricked, dead, damaged, etc)
EDIT: For comparison, in my country a 470 4GB with broken fans and HDMI not working sells for 140€. The normal price range for fully functional 470 is 190-240€.