My last pc was a 3770k (ebay), Msi mpower Z77 (ebay), 16Gb Intel eXtreme Masters edition Patriot 1866MHz (eBay), Samsung 840Pro 120Gb (eBay), Fractal Design R5 (Craigslist), nzxt Kraken x61 280mm AIO (eBay) . The only new items were the psu and gpu. I took that 3770k to 5.0GHz all core at 1.4v, dropped it to 4.9GHz at 1.32v where it sat for the next 6 years, running 24/7/364. Only time it was ever shut down was cleaning and power outages.
So I have no issues with ebay cpus, you get what you get, but I make no guesses at any prior OC or not, I always assume that the cpu has been OC'd (if it's a K squ) and prior owner is lying. Nobody buys a 7700k and tries putting it on a H110m, the cheapest possible mobo, with a stock cooler stolen from a lesser cpu.
There's too many ppl who worry about possible OC and possible damage etc that sellers almost always claim 'no OC', just to alleviate worry. Given 2 identical cpus, one says no OC and the other says was OC'd for several years, which would you buy? The OC seller will have to drop prices to get interest, the non OC seller can raise prices because his is obviously undamaged and perfect as new.
Ignore sales tactics and hype, assume any K squ was overclocked at least partially or for some time period. To me, the seller with a 7700k on a H110m mobo and Intel stock cooler is shifty. That cooler is way underpowered for such a power hungry cpu, no way temps were anything less than 90's while gaming or heavy loads. I'd rather trust the guy selling, who claimed some OC with a Z170 and 240mm liquid cooler. At least I have some clue as to the history, not blind faith.
You could get an intel 3570k/3770k with a boxed stock cooler. It wasn't long after that that Intel decided to stop giving away coolers on K squ cpus, as ppl either stuck them on ebay or threw them away, wasting Intels profit.