The seller was the one who said engineer version. The only thing I'm concerned about is how often, how long it was used.
I kind of feel weird about the situation. Still unsure what I'm going to do.
Wait... you said you found out later it was an ES?
You mean you bought it, then a few days after the seller emailed you and said "hey man, the cpu you bought is an ES."
Maybe by "engineering version" he meant it was pulled out of a workstation or something.
As for how long it was used, or whatever, don't matter none, these modern CPU's, (even the CPU you have is modern) will run 10, 15 maybe 20 years before they just give up the ghost. Nothing to worry about. Assuming you don't overvolt it or something.