According to the law, a contract is invalid under any of the following circumstances:
 1、 One party entered into a contract by means of fraud or coercion, thereby harming the interests of the State;
 2、 Malicious collusion harms the interests of the state, the collective or a third party;
 3、 Cover up illegal purpose in legal form;
 4、 Damage to social and public interests;
 5、 Violation of mandatory provisions of laws and administrative regulations.
 Therefore, in the housing sales contract, if the situation is summarized above, the contract is invalid. To be specific, in case of any of the following circumstances and other legal circumstances that are consistent with the invalidity of the contract in the housing sales dispute, the housing sales contract is invalid:
 1、 After concluding the contract, the seller mortgaged the house to a third person without authorization;
 2、 After concluding the contract, the seller sold the house to a third person;
 3、 The developer has not obtained the pre-sale qualification;
 4、 The developer intentionally conceals the fact that the house sold has been mortgaged;
 5、 Other circumstances prescribed by law.
 Legal basis:
 Article 143 of the Civil Code Civil legal acts that meet the following conditions are valid:
 （1） The actor has corresponding civil capacity;
 （2） True declaration of intention;
 （3） It does not violate the mandatory provisions of laws and administrative regulations, and does not violate public order and good customs.