It’s an illegal sale as registration is compulsory. You need to seek noc and indemnity bond yo secure from any loss or claim from earlier buyers in this case

Registered sale deed is necessary to confer clear and marketable title to flat 

2) the agreement between builder and A and with B 

Is not registered 

3) don’t purchase the flat 

The seller has to execute the registered sale deed in your favour.

For that the seller should have clear and marketable title to execute a registered sale deed in favour of the prospective buyer.

If the vendor is not having clear and marketable title on his name by a registered sale deed in his favour, then he should take steps to acquire that.

You as a buyer cannot get the registration done in favour of the seller to enable him to transfer the property in favour of you by a registered sale deed.

If you still want to go ahead with your proposal to buy this property by hook or crook then you will be the loser because it will be an invalid sale and you cannot sell or legally claim title to the property.

- Since, there is no registered owner of the said flat including A & B , then they cannot sell and transferred the flat in your name legally. 

- Further, as the builder is already traceless, then the ownership of the flat is still in his name or the owner of the land.

- However, A or B can file a declaration suit before the court for declaring himself as the legal owner of the flat 

1. Any immovable property valued more than Rs. 100/- requires registration of the property as per Indian Registration Act 1908. How can you say that registration became mandatory for flats in 2005.

2.  Obtain Declaration Deed from the seller and get the property registered in your name.

Dear client,  

Since the flat is unregistered, you cannot directly transfer ownership to your name. In order to get the flat registered in your name, you will first need to get the ownership of the flat established legally.

You should hire a lawyer who can help you with the process of establishing the ownership of the flat. The lawyer can help you to trace A and get a sale deed executed in your favor by A. You will then need to get the sale deed registered at the local sub-registrar's office.

If A is not traceable, your lawyer may suggest filing a suit for specific performance of the contract against A, which would require proving the existence of the contract of sale between A and B. You may also need to approach the authorities to revive the builder company and/or locate the legal heirs of the company, who may be held liable for the transaction.

In any case, the process may involve considerable legal work and documentation, so it is important to seek professional help to ensure that all legal requirements are met.