A relic is an object of religious veneration, especially a piece of the body or a personal item of a saint.

First Class Relics

Actual part of a saint (a bone, a hair, a limb, etc.)

Second Class Relics

An item that the saint wore (a sock, a shirt, a glove, etc.)

Third Class Relics

The Third Class Relics above fall into 2 categories. The first category is a piece of cloth touched to the body of a saint. The second category is a piece of cloth brought to the shrine (or site of the vision) of the saint.

It is prohibited by the Catholic Church to sell First and Second Class Relics. When the church prohibits the selling of "sacred relics" it is referring to First and Second class relics. It is not referring to 3rd class relics. It is not prohibited by the church to sell Third Class Relics.