Buying used is good if it's from a trusted seller, or if you are buying a locked i5 (which means it can't be overclocked so it couldn't be damaged in that way). Buying used is pretty much your best bet.
I was thinking about the i5-3470 because it's something like 220 euros, the i5-3570k is pretty expensive, 290 euros so i don't know Feel free to suggest more!!
Don't recommend to buy the used cpu except you know the seller. otherwise don't buy it because you don't know what he/she did like don't oc the cpu properly. But it is good deal for the used one.
Buying used is good if it's from a trusted seller, or if you are buying a locked i5 (which means it can't be overclocked so it couldn't be damaged in that way). Buying used is pretty much your best bet.