What is the ruling on buying an apartment in a building owned by a man known to have illicit wealth?

If the man himself is of unlawful origin (such as from zina), there is no harm in buying the apartment from him. However, if the origin of his money is unlawful, then if his money is mixed, dealing with him is permissible, though disliked, according to the majority of scholars. This is unless he bought the building with the unlawful money itself, in which case buying it from him is forbidden. Similarly, if all of his money is unlawful, then dealing with him entirely is forbidden.