Honestly, I would never go for a cpu that's been overclocked before. You never know if the person did it safely or not. How much is it selling for?
Besides, Haswell is coming out in a couple weeks. I've seen the i5-3570k on sale at newegg more than normal lately. It might be safer to pick up a new one that's on sale. If there's a microcenter nearby - I would check that place out to see what kind of price you can get there.
And you never know whether its been overclocked or not in any case, always assume its been clocked. For used gear you could join a private computer forum like overclock.net if you are in the US they have reliable traders and great deals most of the time, way more reliable than the eBay.
4.2Ghz is a relatively low overclock for an i5 3570K. In fact it can usually be done without increasing the CPU voltage.
Now for my problems. The only watercooler Scythe ever made is OLD. Like Pentium 4 Socket 478 old. Like obsolete in 2005 old. And as far as I know it would not work with Ivy Bridge. So the seller might have tried to use an incompatible cooler with that CPU and fried it.
You will also not have a warranty with a CPU bought from ebay.
So if you just have the money laying around and want a new CPU to play with and you are aware of the risks and prepared to lose that money then go for it. If it's something you need to save for then save a bit longer and buy new.
Also check the sellers rating on ebay. If he has a very high rating and good reviews that would be another mark in favor of buying it.