1, legal relationship analysis:

The topic is buying and selling relationship, but there are many relationships involved.

(1) Zhang's tort constitutes a legal relationship of tort debt. At the same time, Zhang is a minor, and his guardian should bear corresponding responsibilities.

Zhang sold his watch to Lin. Zhang is a person without capacity for civil conduct, and his behavior is an invalid legal act. Zhang's guardian has the right to revoke it. Lin knows that Zhang is a person without civil capacity, and his purchase behavior is actually flawed and subjectively wrong.

(3) The buying and selling relationship constituted by Lin's buying and selling behavior. It mainly involves the transfer of real right, that is, the responsibility of the owner who sells the subject matter.

2. Whether the plaintiff is qualified.

It is suggested to combine specific laws and regulations for in-depth analysis.