Doing an NDA to "keep the sale off public record"

[Steve S.]

I've been doing some research to try to determine how much my aqcuisition price for my rentals affects the amount of taxes I will be paying and have largely gotten nowhere.  I can assume takes will be based on whatever i payas a semi-worst case scenario, but even that might not be worst if the values keep going up.

That said, I came across some sites where the notion of doing an NDA to keep the sale off public record was mentioned.

This sounds totally illegal.  Anyone have any insights here?