By the skin of their teeth 點解?

by the skin of one's teeth是剛好，勉強（僅僅）的意思
除了這句以外，翻譯這句時，還要留意would have been，would +have +been是一種特別的用法，是臆測的說法，如：I think Tom would have gone（我認為湯姆當時已經去了：但湯姆當時有沒有去，說話者未能確定）
所以全句是：我擔保，沒有這物業的買賣，他們當時只能勉強生活。（但因為這事根本沒有發生，故只屬臆測（其實老外的邏緝性非常精密，有時真是要佩服他們的精確））

2007-12-23 22:54:28 補充：
這樣會更合原意：我擔保，沒有這物業的買賣，他們當時（或）只能勉強生活

According to Oxford Advance Learner's Dictionary, 'by the skin of one's teeth' means 'only just'. Following the explanation, it has a sample sentence: He escaped by the skin of his teeth. 他剛好及時逃掉。
Your above sentences should mean the money which they get from the puchase and sale of the related property help them to solve lots of problem, otherwise their financial status at that time was just enough for them to survive.

By the skin of their teeth


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