Read this:
讀讀這個：

If the Deposit is not paid, the Seller has the right to rescind this Agreement by a notice in writing to the Purchaser delivered within 30 days from the date of this Agreement, provided that such rescission right shall not be effective unless and until the Seller does not rescind the Preliminary Agreement for the Property.
如果保證金并未償付，賣家有權于該協(xié)議成立之日起30日內書面告知買家解除該協(xié)議。除非賣家并未解除資產預備協(xié)議，否則該解除行為將不會生效。

Got it? If not, feel free to read it again. And again… and probably again.
你讀懂了嗎？如果沒讀懂，你就再讀一遍，再讀一遍……或許還要再讀一遍。

Got it now? Perhaps you should read it once more just to double check.
現在懂了嗎？或許你應該再讀一遍來檢查一下。

No doubt you're having trouble with this part: "…shall not be effective unless and until the Seller does not rescind…"
毫無疑問你會對這段有所疑惑：“……除非賣家并未解除資產預備協(xié)議，否則該解除行為將不會生效?！?/div>

I think you'll agree that the difficulty is mainly due to the use of multiple negatives – "shall not", "unless" and "does not". It's more of a brain teaser than a practical piece of communication. Unfortunately most of us don't have the time or inclination to work out brain teasers.
我相信你們都會認同難點主要來自多重否定的使用——“將不會”、“除非”和“不會”。這不是實用的交流方式，而是腦筋急轉彎。不幸的是，我們大部分都沒時間也沒興趣去做什么腦筋急轉彎。

So – my advice is simple: Don't use multiple negatives. Write something like this instead:
因此——我的建議很簡單：不要使用多重否定。用這樣的寫法來代替：

If the Deposit is not paid, the Seller has the right to rescind this Agreement by written notice to the Purchaser delivered within 30 days of the date of this Agreement. However, the rescission right is not effective if the Seller has rescinded the Preliminary Agreement for the Property.
如果保證金并未償付，賣家有權在該協(xié)議成立之日起30日內書面告知買家解除該協(xié)議。但如賣家已解除資產預備協(xié)議，該解除行為將不能生效。

I know I'm taking something of a risk pretending I've understood the original correctly, but I'm willing to do so. If my interpretation is wrong, far from showing my lack of intelligence, it proves how bad the writer is at communicating.
我知道這么說有風險，那就是我不一定正確地理解了原文的意思，但我愿意這么做。假如我理解錯了，這除了表現出我智商不夠用以外，更多地是表現了原作者的溝通能力是有多差。

In case I haven't convinced you that multiple negatives are a bad idea, here are a few more brain teasers for you to waste your time on:
要是我還沒有能夠說服你多重否定是個多糟糕的做法，以下是一些更難的浪費你時間的腦筋急轉彎：

The following actions are not excluded from the prohibition.
以下行為并沒有被排除在禁令之外。

No termination will be approved unless the administrator reviews the application and finds that it is not lacking in any requisite materials.
除非管理員查看申請并發(fā)現其并不缺少任何必備材料的情況下，任何終止都不會被允許。