How To Know If You're Being Sold A Low-Speed Processor When Buying Used Or Refurbished Computers

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

The only way to really check to make sure that you're not getting a slower processor than the private seller has advertised is to look at the processor itself. Through software, you can manipulate what the processor speed is showing, so unless you really physically look at the processor chip, you can't be 100% sure that the speed that you're getting is actually the correct speed.