Obviously you're going to want to check it out, make sure it works etc when buying second hand. Overclocking is actually very safe and easy to do these days, they come loaded with auto-shutdown if they reach high temps and/or problems. Although overclocking does technically shorten the life of your chip.. It's not by much. I wouldn't have a problem with buying a used chip as long as it's working and doesn't appear to be damaged. When you install the chip into your new motherboard, overclocks are reverted to stock (as this runs off the motherboard bios, not the cpu). So you can start fresh.
Since it's a K processor and you may want to OC in the future.. If not already, i recommend buying a aftermarket cpu cooler...