Why does my textbook give 売らんがための as the phrase for ____ in この雑誌には＿＿＿誇張表現が多いようだ and not 売るべく? From the usage info they give, either should work as both have volitional verbs before the ___. Is it that 表現が多いようだ is not expressing volition that makes 売るべく not appropriate? Thanks.
売るべく works adverbially, 売らんがための works adjectivally (and 売らんがため without の works adverbially). The modified phrase, 誇張表現, is a noun phrase, so it should be modified by an adjectival expression.
exaggerated expressions in order to sell
exaggerated expressions that should be sold.
べきだ is used to express duty, obligation, something that you should do.
んが為 is used to express purpose, reason, motivation.
So the advertisers will use outlandish expression in order to sell more.
But they are not legally or morally obligated to sell. So 売るべき doesn't make much sense.