There is a well researched article written online, you can read the article here http://timesofindia.indiatimes.com/nri/us-canada-news/NRIs-guide-to-selling-property-in-India/articleshow/9723296.cms
Hey Sumana, An encumbrance certificate is necessary to assure the buyer that the land is free from dues to any legal authority. In case you are not able to produce EC, then you have to go for a lawyer certificate that proves the ownership of the property. Though this certificate is also admissible for a sale.