This piece is called the "Tail Light Side Panel". The part comes with a brand new gasket installed on it.
The left side is BMW part # 63-21-6-933-017
The right side is BMW part # 63-21-6-933-018
This part is for the coupe (Ci) model - if you have a convertible, it's a separate part. The coupe part fits the 323Ci, 325Ci, 328Ci, 330Ci, and M3.