英语翻译Vanity is a constant; people will only start shopping mo

when they can’t afford not to.怎么翻译?
Vanity is a constant; people will only start shopping more sustainably when they can’t afford not to.最后那句怎么翻译成只有在买不起的情况下,不是有两个否定吗,不该是肯定的吗?
when they can’t afford not to..
中文意思是:
当他们不能不(这样做)时.
can’t afford not to 是不能不.(是一句固定配搭)
但在这个句子里却要翻译为"无法错过".
整个句的意思是:
虚荣是一个常数,当人们在无法错过的情形下将会更加开始大量购物.

不赞同采纳的答案...
这句话完整的表述应该是：
Vanity is a constant; people will only start shopping more sustainably when they can’t afford not to (shop sustainably).
虚荣心很常见；人们只有在负担不起（非持续性购物方式）时才会采取更具持续性的购物方式。