Correcting a faasid marriage

Assalamualaykum, According to Imaam Malik’s ruling,witnesses for a marriage are not required,Does this mean,If a person proposes and the other accepts Nikaah is valid?Is announcing it publicly a condition in order for it be valid according to Imaam Malik’s ruling?I read this type of Nikaah is considered faasid(corrupt),How does one correct a faasid marriage? Shukran

Faasid sales

Mention some of those conditions which will render a sale Faasid?