Mutation of property in name of seller is necessary 

2) registration of flat has to be done in name of seller 

3) then only should you buy flat 

See the seller needs to take permission from housing society just the formal one and can sale the flat through valid sale deed and transfer share certificate in your favour.

I think the proeprty is old you need share certificate and the agreement by which seller recieved the property.

Permission from housing society can be obtained. They shall not be the confirming party.

1. It is a leasehold property for which there can be no convey of title.

2. He can convey the lease right to you if that is permitted in the initial lease agreement he had signed with the leasor. 

3. The lease deed is required to be seen for giving proper advise.

1. The lease deed based on which the said person has taken possession of the said flat is the prime document to be checked.

2. The property is required to be searched and search report obtained.

3. The possession letter, mutation certificate and the share certificate is to be collected.

4. The agreement based on which the said society has been formed is also required to be verified.

1. You shall have to obtain a letter from the secretary of the Society confirmingt that there is no dues pending in favour of the current lease holder and also that the Society has no objection in your buying the leqase right from him.

2. Society will not be the confirming party. It will isse NOC only.

If the vendor has no title to the property then it would be illegal for him to sell the property and it is absolute waste for you to buy this property from a person who has no title  or rights or interests in the property.

Let him produce the title documents to prove his title to the property after which you can decide about buying it.

Firstly you dont venture into purchase of a leasehold property.

Ask him to convert the same to freehold property after which you may obtain all the tax paid receipts and other relevant documents standing on his name to prove his marketable title to the property.

You may consult a local advocate and obtain a legal opinion which will cover all the documents that are required to be produced by the seller to the buyer.

Mutuation should be in his name else you will have problems to transfer or sell. 

Housing society has nothing to do with the sale of individual's property except if they would be required to give NOC.

 Always buy property that is on clear title. Always deal with the official and registered owners. 

You need NOC from society 

2) society need not be confirming party 

1) check whether seller has original documents of title 

2) obtain NOC from society 

3) no dues certificate 

4) check whether seller has original share certificate 

Share certificate is not a document of title. Title to a property originates from the sale deed. Do not buy the property if he does not have a sale/conveyance deed.

1. As per the information mentioned in the present query, makes it clear that the seller wants to sell the leasehold flat to you.
2. There has been no law, which prohibits of selling the leasehold property to some one else.
3. You just have to be ensured that the seller should have all paper relating to his ownership over the same and no objection certificate from the society.

If the seller is not a promoter member of the society then without a registered sale deed you can not get a clear titile.

The rile of the society in West Bengal is not so important.

Unless all the papers are seen it is difficult to advise further.