Complex sentence: Some DC-9s including stretched DC-9-31 models were bought new while others including DC-9-14 jets were purchased used from Delta Air Lines which had acquired these aircraft new.
Split into simpler sentences: Some DC-9s including stretched DC-9-31 models were bought new while others were purchased used. These used jets included DC-9-14 models which were acquired from Delta Air Lines which had purchased these aircraft new.