Many efforts have been made to develop the market for imported goods.
Literal
Import-goods market [object-を] develop many efforts [subject-が] have-been-made.
なされてきた is the passive of なす ('to perform') + てくる, meaning 'have been made over time up to the present.' 開拓する (かいたくする, 'to pioneer, develop') is a formal verb for market development. The passive frames the efforts as ongoing institutional action.