Under Philippine law, co-ownership is governed primarily by the Civil Code (Articles 484 to 501). When a co-owner sells the entire property without the consent of the others, or when title is fraudulently transferred, the legal landscape shifts into a complex interplay between the Civil Code, the Property Registration Decree (P.D. 1529), and various remedial laws.

1. The Principle of "Nemo Dat Quod Non Habet"

The foundational rule in co-ownership is that each co-owner has full ownership of their ideal (undivided) share. However, they do not own a specific, physical portion of the property until partition.

Article 493 of the Civil Code: A co-owner may alienate, assign, or mortgage their ideal share. But the effect of the sale is limited strictly to the portion which may be allotted to them in the division upon the termination of the co-ownership.
Legal Consequence of Unauthorized Sale: If a co-owner sells the entire property without consent, the sale is not void ab initio (from the beginning) regarding the seller's share, but it is unenforceable or void regarding the shares of the non-consenting co-owners. The buyer merely becomes a new co-owner of the seller's ideal share.

2. Remedies Against the Co-Owner/Seller

When a co-owner disposes of property without authority, the aggrieved co-owners have several avenues for redress:

A. Action for Partition (Article 494)

Since the buyer only steps into the shoes of the selling co-owner, the remaining co-owners can demand a physical division of the property. If physical division is not possible, the property may be sold and the proceeds distributed.

B. Legal Redemption (Article 1620)

If a co-owner sells their share to a third party, the other co-owners may exercise the right of legal redemption.

Period: This must be exercised within 30 days from written notice by the vendor or seller.
Purpose: To keep the property within the original group of co-owners and prevent the entry of strangers.

C. Accounting and Damages

The aggrieved co-owners can sue for an accounting of any proceeds or fruits (like rent) received by the selling co-owner from the unauthorized sale, along with moral and exemplary damages under the Human Relations provisions of the Civil Code (Articles 19, 20, and 21).

3. Remedies Against Fraudulent Transfer of Title

If the sale resulted in the issuance of a new Transfer Certificate of Title (TCT) in the buyer's name through fraud (e.g., forged signatures on a Deed of Sale), the following remedies apply:

A. Petition for Annulment of Sale and Cancellation of Title

This is the primary civil remedy. The plaintiffs seek to declare the Deed of Sale void and pray that the Register of Deeds cancel the fraudulent title and reinstate the original co-ownership title.

B. Action for Reconveyance

If the property has already been registered in the name of the fraudulent buyer, an action for reconveyance is the proper remedy.

Ground: Based on the principle that the registered owner is merely holding the property in an implied trust for the rightful owners.
Prescription: * If based on fraud: 4 years from discovery.
If based on an implied trust: 10 years from the issuance of the title.
If the plaintiff is in actual possession: Imprescriptible (the action does not expire).

C. Recovery from the Assurance Fund

Under P.D. 1529, if a person is deprived of land or any interest therein through the operation of the Torrens system (due to fraud or errors by the Register of Deeds) and is barred from bringing an action for recovery, they may file an action against the Assurance Fund maintained by the National Treasury.

4. The "Innocent Purchaser for Value" (IPV) Defense

A critical hurdle in these cases is the Mirror Doctrine. A buyer is generally entitled to rely on the face of the Torrens certificate of title.

Protection of IPV: If the buyer purchased the property in good faith, for value, and without notice of the defect in the seller's title, the sale may be upheld to protect the integrity of the Torrens system.
Exceptions:
Bad Faith: If the buyer knew of the co-ownership or saw indications of a flaw (e.g., the seller was not in possession), they are not an IPV.
Forged Deed: While a forged deed can be the root of a valid title (if it passes to an IPV), it is generally null and void between the immediate parties.

5. Criminal Liabilities

The unauthorized sale and fraudulent transfer usually involve criminal acts punishable under the Revised Penal Code:

Estafa (Article 316, paragraph 1): Explicitly penalizes any person who, pretending to be the owner of any real property, shall convey, sell, encumber, or mortgage the same.
Falsification of Public Documents (Article 172): Applicable if the seller forged the signatures of the other co-owners on the Deed of Absolute Sale or used a falsified Special Power of Attorney (SPA).

Summary Table of Remedies

Remedy	Objective	Primary Legal Basis
Legal Redemption	To buy back the share sold to a stranger.	Art. 1620, Civil Code
Action for Partition	To end the co-ownership and get a specific portion.	Art. 494, Civil Code
Action for Reconveyance	To return the title to the rightful co-owners.	Art. 1456, Civil Code
Annulment of Sale	To declare the contract of sale void.	Civil Code / Rules of Court
Criminal Complaint	To imprison the perpetrator for Estafa/Falsification.	Revised Penal Code