Suppose Mr.X is the title owner of the property and he has the sale deed and other relevant documents on his name then it is easy for him to sell his property and convenient for you to buy it.
But in some cases people sell their property on GPA or Agreement to Sell, but these documents are not considered as valid documents for sale of property since the owner cannot be considered as the proper and legal owner and the sale through GPA is not considered viable and valid for title transfer.
In the case of GPA it is only for a specific task to be done, suppose the owner of the property is living abroad and cannot come to collect rent for the property, or the owner is sick to come for the registration of sale deed then they can provide GPA to their relative to complete the task. But you cannot do Title Transfer through GPA.
GPA although can further sale or final sale to a third person and can be the new owner of the property.
It is very important to get a Sale Deed registered at the Sub Registrars Office but it is very important to note that a Title Transfer cannot be done through GPA or Agreement To Sell. GPA is only legal for completion of some specific tasks only. Further you should that even Banks will not give loans to people citing their ownership through a General Power Of Attorney.