To whom does a necklace found by a man inside a car offered for sale for its parts belong, knowing that the seller buys inoperable cars and allows buyers to remove parts themselves, and does the answer differ if the finder does not return it to the shop owner for fear that he might seize it?

The necklace must be given to the shop owner, as the default assumption is that whatever is found in his premises belongs to him. If it does not belong to him and its owner can be identified, then it must be delivered to them. Otherwise, it should be given in charity on their behalf. If its owner cannot be identified, then it falls under the ruling of a luqatah (lost and found item), and it must be announced for a year. After that, the finder may dispose of it while guaranteeing it to its owner if they are found. It is advisable to give it to the shop owner to announce it if it is highly probable that this will help it reach its owner.