Final answer:
In a real property installment sales contract, if a buyer defaults and a quitclaim deed is used for foreclosure, the seller must sign the quitclaim deed.
Step-by-step explanation:
In the case of a buyer defaulting on a real property installment sales contract that was recorded by the seller, a quitclaim deed can be used to foreclose. When using a quitclaim deed, the party who must sign is the seller, also known as the grantor. The grantor is responsible for signing and transferring any ownership rights they have in the property to the party receiving the quitclaim deed, known as the grantee.