No it's not trespass, No forceful eviction allowed in US

This is a common misconception.  Trespasser are not "in possession" of land, they merely are present on it.  A former owner, or a tenant (even in violation of agreements) still have possession of land, that gives them certain rights.

You can call police to remove a trespasser, because the property right is clear, the police knows the trespasser doesn't belong there.  If the owner calls police to evict a tenant or a former owner (such as after foreclosure), the police are not trained to determine who has right to be in the land (lease agreements, closing documents), therefore they are trained not to do anything.

To evict a former owner or a tenant, the owner has to petition the court for eviction.   For this reason, our friend in this case would have a quicker solution by withholding sales proceed to the former owner, instead of going to court.