The purchaser of advance sale of commercial housing is usually on a social weak position,often is the injured party in practice,so how to protect their interests has become a legal issue.

Retained ownership installment contract has validity to both the purchaser and the seller, including the transfer of buyer s enjoyment and expectant right, custodial obligation of subject matter, retained ownership, recall and exercise rights of buyer and both parties disposition of subject matter, etc.

The protection of the benefit of the buyer can be realized by endowing the reasonable purview of the redemption, claim for resell and, accession, to the buyer.

The problem about the protection of the buyer’s benefits in sale by installment payment is a subject worthy of deep and inclusive research.

This paper mainly illustrates how to protect the benefits of prior purchaser in follow respects: the efficacy of contract for house dual sales ,the right of cancellation of the prior purchaser, the compensatory spheres of seller in breach of contract, the burden of risk of seller.

Multi-buy-and-sale of property means the owner of the property sells his house to more than one buyer and signs more than one contract, which forms more than one buy-and-sale relation and causes disputes easily, damaging the buyers interests.