It is also illegal to sell them as Intel never sells them in the first place, all ES chips are property of Intel on loan to their various platform development partners. (like ASUS, Supermicro, etc)
Now that you know it is an ES, regardless whether or not the "seller" has any legitimate claims as to why it should be viewed as superior to a retail chip you now know the seller is trying to sell you something they don't technically own (even if they thought they purchased it legitimately from someone else, buying a stolen car doesn't make it not stolen any more).
I wouldn't touch it.
That is a very good point, and as mentioned before I assume thats why the etching is taken off. Similar to that of filing down the serial on a stolen gun.
I've also read elsewhere that if you turn in ES chips to Intel they will give another chip back to you of equal or higher value. This does not seem right, but I thought I'd just put it out there.
