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.
A common problem in the Philippines is buying property but the title is still registered under the name of deceased persons. In short, you are dealing with the heirs of the owners of the real property.
The problem with this is that there are many heirs of the property and it may be that it becomes very difficult to assemble all the heirs to agree to sell and sign the document.
If you are just dealing with one heir, you should make sure that all the heirs agree to sell to you. This is to avoid headache and have peace of mind. Just one heir cannot sell the entire property to you. In that case you are only buying his share, meaning you become a co-owner with the heirs. But what you are buying, really, is headache.