When Seller needs to Approve the Buyer's Assignment of the Purchase-and-sale Agreement (a) What is the legal rationale for the default rule that purchasers are allowed to assign real estate purchase-and-sale agreements without the consent of their sellers? (b) Why is a purchaser presumed to need the seller's consent to assign a realty sales contract when the contract contemplates that the seller is to finance the sale by taking back a purchase money mortgage? (c) Why might a seller be well-advised to bar assignment of a purchase-and-sale contract without the seller's prior written consent even when no vendor-financing is involved? After all, it is well established in contract law that if the purchaser assigns and her buyer defaults, the purchaser remains liable on the original contract