▁and ▁bringing ▁production ▁under ▁planned , ▁collect ive ▁control ▁by ▁the ▁freely ▁associated ▁produ cers . ▁ ▁Mod ification ▁in ▁the ▁world ▁market ▁Marx ▁believed ▁that ▁the ▁operation ▁of ▁the ▁law ▁of ▁value ▁was ▁not ▁only ▁modified ▁by ▁the ▁capital ist ▁mode ▁of ▁production , ▁but ▁also ▁in ▁the ▁world ▁market ▁( world ▁trade , ▁as ▁contrast ed ▁with ▁the ▁home ▁market ▁or ▁national ▁economy ). ▁ ▁The ▁main ▁reason ▁for ▁this ▁was ▁the ▁existence ▁of ▁different ▁levels ▁of ▁the ▁intensity ▁and ▁product ivity ▁of ▁labour ▁in ▁different ▁countries , ▁creating ▁for ▁example ▁a ▁very ▁different ▁cost ▁structure ▁in ▁different ▁countries ▁for ▁all ▁kinds ▁of ▁products . ▁Product s ▁that ▁took ▁ 1 ▁hour ▁of ▁labour ▁to ▁make ▁in ▁country ▁A ▁might ▁take ▁ 1 0 ▁hours ▁to ▁make ▁in ▁country ▁B , ▁a ▁difference ▁in ▁production ▁costs ▁which ▁could ▁strongly ▁influence ▁the ▁exchange ▁values ▁real ised ▁in ▁the ▁trade ▁between ▁A ▁and ▁B . ▁More ▁labour ▁could , ▁in ▁effect , ▁exchange ▁for ▁less ▁labour ▁intern ation ally ▁( an ▁" une qual ▁exchange " ▁in ▁value ▁terms ) ▁for ▁a ▁prolong ed ▁time . ▁In ▁addition , ▁the ▁normal ▁rate ▁of ▁sur plus ▁value ▁could ▁be ▁different ▁in ▁different ▁countries . ▁That ▁makes ▁a ▁huge ▁difference ▁not ▁only ▁to ▁profit ability , ▁but ▁to ▁the ▁ability ▁to ▁sell ▁products ▁at ▁compet itive ▁prices . ▁ ▁So ▁trad ers ▁would ▁try ▁to ▁use ▁this ▁differential ▁to ▁their ▁advantage , ▁with ▁the ▁usual ▁mot to ▁" bu y ▁cheap , ▁sell ▁dear ".