How to sell the property after mothers death

Hi Team, I am Dilles, My mother bought 42.5 Cents of land in Vizianagaram district in 2003. She passed away on 11.10.2011 and I am the legal hearer for my mother and I have the certificate as well. Now I have given an agreement to the local person in the same place to sell my land. Another person is coming up with an agreement on Rs. 50 stamp paper as some part of the land has been sold to him on the agreement dated 27.09.2011. I dont know exactly my mother given an agreement to him or it is a fake document. one more thing is at that time I was 27 Year old. Could you suggest me how can I proceed to sell my property. Any legal implication on me if I sell the property by ignoring the agreement shown by the third person.