First of all since the property has not been registered on the name of the seller, there should be a tripartite agreement between the seller,buyer and the builder.
If the original agreement is missing then the seller should indemnify the buyer against the future litigation or loss in this regard besides giving an affidavit to this effect. He should also give a copy of the complaint that has been lodged with the police in this regard.
Also obtain an acknowledgment from police and get a public notice published in this regard in a local newspaper.
After all these are completed you may obtain a legal opinion from a local lawyer about the genuineness of this property before buying it.