I don't know if CEX is a UK-only thing, it's a chain store that holds used games, mostly of the previous 2 gens (Xbox 360, Wii, etc.) but it has some old games.
CEX is a UK only thing. Last I saw them holding cart based Nintendo, they wanted over Ebay prices. The problem with them is they take anything and dont test so ive had agood few dud items from them long ago.
Also I believe CEX name is Computer EXchange not Central Exchange...