How can the old owner get a writ of possession ordering the property be turned over to him, when he no longer owns the property?

Not necessarily if the sale was handled correctly the new owner assumes the rights of the old owner.

Yes. The new owner should get a writ of possession, but maybe they agrees as part of the terms of the sale, the prior owner did it.

There are two ways that this could happen. One, the sale of the apartment building included a clause that required the former owner to complete the summary proceedings process. Two, the new owner assigned the right to remove you and possibility to collect past due rents. In other words, the past due accounts still belonged to the old owner. There is no legal right to require the new owner to start the process over again.

Not necessarily. Don't you think you should move out and find a new place?

Probably not. The old owner may have sold the building subject to his evicting you or may be operating on behalf of the new owner as his/her agent. When the judgment was obtained, the old owner still owned the building; the steps that occurred after the judgment were merely the enforcement of that judgment.

Probably not. First, the Judgment was entered prior to the time of the sale, and the old property owner is entitled to take those steps required to enforce the Judgment. Second, the sale of the building very well may have contained a contingency that the old owner would continue to take the necessary steps to regain possession and that the new owner was buying the property with the expectation that this would occur. In any case, the new owner is not required to re-start the Unlawful Detainer process in order to have you removed. My best advice? Move out! If the Sheriff Department comes to lock you out you will have a very limited amount of time during which to remove your personal property before changing the locks, and then you will have to arrange a time to obtain the rest of your stuff. You don't want it to come to that it's much easier for you if you just leave before law enforcement arrives with a locksmith!