Only negotiate when you are certain to profit
On the other hand, several sites list this one as Rule 42:
What's mine is mine, and what's yours is mine too.
Other websites disagree, not listing a Rule 42, as they aren't fully disclosed within the Star Trek: Deep Space Nine canon. The Rules of Acquisition have their own dedicated Wikipedia entry.
Here's video of the rules which are stated during the TV series episodes:
Alas, Rule 42 isn't among them.