BloodStone

If I remember correctly, those mean (respectively): 'Engineering Sample' and 'Qualifying Sample', not sure what QQBY is.

It's an engineering sample and they vary wildly. Most don't oc too well but some do. However these cpu's do have a high risk of other issues and were meant to be destroyed.

A QS and a ES chip are two very different things. 
 
QS= Qualification Sample - These are review units typically, they are nearly as good as the consumer grade product. You do run the risk of someone who had this before overclocking it excessively and doing damage however. 
 
ES= Engineering Sample - These vary wildly in quality. Early versions can be buggy messes, overclock poorly, and just not work right at all. Later versions can be very close to the retail chip with just lower stock clocks. These are riskier. The chip you are looking at is an Engineering Sample not a Qualification Sample despite the misleading listings online. 
 
This thread provides a bit more detail about the particulars of the one you are looking at. 
 

I run a QS Xeon 1660v3 (similar to i7 5960x) that I knowingly bought with a bad memory controller (only 4 of 8 ram slots will work with it) but I paid $112 so a lot less money involved. 
 
You take a pretty big chance with these chips having issues even more so with ES CPUs. When it's cheap enough these are worth taking some risk and are fun to play with. 
 
At $340 these are tough to reccomend. The Ryzen 3700x is less than that with a warranty and no risk. You can also try to find a steal of a deal on a 7820x delid it and have a very good CPU. I've seen those occasionally on Reddit hardwareswap and other places for $250-300. 
 
Now, if you find one of these 9900k unlocked ES chips at $250... Well that's probably worth taking the chance.