Resale of unregistered flat

How could bank give loan without registration,, it is highly improbable

or mortgage would be there that too requires registration.

find out what type of agreement was entered into between the builder and the purchaser if they say none they may be lying.

the best option is to enter into an agreement with the builder and make the first purchaser as the confirming party and register it.

Get the loan details from the bank and mention it as the part of consideration and pay it to the bank

accordingly answers to your questions follow

1. Yes you can buy.

2. Once he has paid the money he is the actual owner. Registration does not convey title it sort of perfects it.

he can sell it to you but then first agreement would be required to be at least stamped with stamp duty while under making the purchaser as confirming party, you may not be required to pay stamp duty.

3. It is not valid if any agreement exists with the builder and if no then it could be.

4. He cannot cancel the allotment once heh as received the money .

5, yes some amount may be payable because builder may demand extra payment, because you require. Him to come to register it

6. I do know about service tax implications... 

I Meant I DO NOT KNOW ABOUT service tax implications.......