Could somebody explain the meaning of the sentence to me?
I think you are meant to fill something like this：


  今天的菜非卖掉不可， （否则）到明天就没有人买了。 


到明天就没有人买了 here implies that the vegetables might go stale and no body wouldn't buy it. That's why you have to make sure you sell them all out today -- 今天的菜非卖掉不可.