Can we give "Not Leak" guarantee for houses we inspected?

Very Tough question.

To answer that question, I would like to refer to a particular CASE:  TRI-2009-101-000060
In this case a pre-purchase inspector is found to be liable for his report and ordered to pay more than $20000.

So, it has happened. We have to be careful about reports we issued. The key point here is not "guarantee". Focus of judgement here is whether the inspector is negligent.

Our opinions have to be "fair" and "independent" and based on "competency".

In other words, if my opinion is fair and independent, but I'm not competent enough to carry out house inspections, I may be found negligent.

Or, if my opinion is not fair and independent, even if I'm competent enough, I can still be found liable for negligence.

But, if my opinion is fair and independent, and I'm competent enough to carry out house inspections, then I may not be liable even my opinion is proved to be wrong.

So, when I say "Not leak", this is to be regarded as a conditional guarantee. Though I have to say, very hard to find me saying "Not leak".