Definitely not stolen. It seems the only ones Intel cares about are the unmarked ones..
"Last Reviewed: 13-Apr-2016
Article ID 000005719
This document addresses questions about Intel® Engineering Sample Processors.
How do I identify Intel® ES processors?
The easiest way to tell if you have an Intel ES Processor is to look at the processor topside markings. If you have a 4 or 5 digit Qspec listed like the one pictured below, you have an engineering sample. A production processor will have a 5 digit sSpec, usually beginning with S, such as SLB9L on the topside markings.
If no ink or laser markings are found, the processor may be an "unmarked" Intel ES Processor.
Contact Intel if you are in possession of an unmarked Intel ES Processor."
http://www.intel.com/content/www/us/en/support/processors/000005719.html?wapkw=intel+confidential
Technically they can request you to return them, but generally don't. If someone has a large quantity of them, that may be a risk, but the average ebay buyer who buys one is at little risk of this happening.