מילתא דאמר רחמנא 'לא תעביד' - אי עביד לא מהני מידי
Something regarding which the Torah said "don't do [it]" - if one does it it has no effect.
(This topic is discussed in T'mura 4b.)
What does this rule mean and in what situations does it apply?
He explicitly contrasts this rule - which he applies in explaining the Ramba"m's position that there is no punishment for conspiracy to unlawfully obtain the possessions of another - with the case of forced sale. The latter is used elsewhere as an example of an illegal act that, once committed, has legal validity nonetheless. Furthermore, are there not other cases in which a person acts illegally but is still able to effect a legal effect, such as sales predicated on misinformation? There is obviously a distinction I am missing.