Read this:
读读这个：

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.
如果保证金并未偿付，卖家有权于该协议成立之日起30日内书面告知买家解除该协议。除非卖家并未解除资产预备协议，否则该解除行为将不会生效。

Got it? If not, feel free to read it again. And again… and probably again.
你读懂了吗？如果没读懂，你就再读一遍，再读一遍……或许还要再读一遍。

Got it now? Perhaps you should read it once more just to double check.
现在懂了吗？或许你应该再读一遍来检查一下。

No doubt you’re having trouble with this part: “…shall not be effective unless and until the Seller does not rescind…”
毫无疑问你会对这段有所疑惑：“……除非卖家并未解除资产预备协议，否则该解除行为将不会生效。……”

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.
我相信你们都会认同难点主要来自多重否定的使用——“将不会”、“除非”和“不会”。这不是实用的交流方式，而是脑筋急转弯。不幸的是，我们大部分都没时间也没兴趣去做什么脑筋急转弯。

So – my advice is simple: Don’t use multiple negatives. Write something like this instead:
因此——我的建议很简单：不要使用多重否定。用这样的写法来代替：

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 However, the rescission right is not effective if the Seller has rescinded the Preliminary Agreement for the Property.
如果保证金并未偿付，卖家有权在该协议成立之日起30日内书面告知买家解除该协议。但如卖家已解除资产预备协议，该解除行为将不能生效。

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.
我知道这么说有风险，那就是我不一定正确地理解了原文的意思，但我愿意这么做。假如我理解错了，这除了表现出我智商不够用以外，更多地是表现了原作者的沟通能力是有多差。

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:
要是我还没有能够说服你多重否定是个多糟糕的做法，以下是一些更难的浪费你时间的脑筋急转弯：

The following actions are not excluded from the prohibition.
以下行为并没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.
除非管理员查看申请并发现其并不缺少任何必备材料的情况下，任何终止都不会被允许。