Law on Co-owner selling his Undivided Share
A co-owner in a joint property can sell his undivided share of property to anyone. But he cannot sell the specific piece of land with marked boundaries from the joint property.
A selling of a share of property by co-owner in essence means the buyer steps into the shoes of his seller. The buyer is clothed with all the rights and liabilities of his transferor. So, the buyer becomes as much a co-owner as his transferor was before the property transfer.