How do I know if a private seller is selling me a processor with a lower speed?

How do I know if a private seller is selling me a processor with a lower speed?

Ali Nasri (Computer Specialist, Mac 911 Inc.) gives expert video advice on: Are refurbished computer systems a good idea?; Do I need to inquire about the brand and speed of the internal CD/ DVD drive?; Why are risks of gray market parts? and more...