confidential cpus are typically engineering samples, usually given to motherboard companies to work on bioses for the cpu generation and meant to be returned to the company after done. engeering sample cpus can be missing features, lower clocks and such that the retail varient may have.

as for geneeation of cpu, its easier to find out via what motherboard socket it is

on thecontext of finding second hand, i wouldnt say its common, but can happen. some chinese companies sell dirt cheap ES cpus for basic computing.