No, a co-owner is entitled to a proportionate share in the land but he is not the owner of a specific portion of land. He can be an owner of a specific piece of property only when the property has been partitioned.
In this sense, he cannot sell a specific piece. Thus, when a buyer purchases a specific piece of land from the co-owner, it must be with the consent of all the other co-owners.
Otherwise, the buyer merely purchases the pro indiviso share of the seller/ co-owner.