Vanity is a constant; people will only start shopping more sustainably when they can't afford not to.
这里的后半句when they can't afford not to该如何理解?
can't afford to do/not to do. 无法承受做或不做某事的后果。你的句子中省略的不定式就是前面的start shopping....当他们无法承受不这样做的后果时,将只好这样做。
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