In whose name should the stamp paper be purchased? Can the client exercise the document on a stamp paper purchased by an Advocate?
Stamp papers should be in the name of one of the parties who has signed the documents. Therefore, the stamp paper should be purchased in the name of one of the parties who would be signing the instruments. If the stamp paper has been purchased in the name of an Advocate, C.A., etc. then such instrument shall be treated as an instrument not duly stamped and shall be inadmissible in evidence.

In case of resale of the flat who has to pay the stamp duty, the purchaser or the seller?
The parties can themselves decide who shall pay the stamp duty. If nothing is mentioned in the agreement then, if the transaction relates to resale of flats then the stamp duty will have to be paid by the purchaser.

Will Stamp Duty have to be paid if there is a deed of family settlement and a flat is transferred amongst family members?
Yes, As per the provisions of Stamp Act, Stamp Duty will have to be paid on a deed of Family Settlement.

Will Stamp Duty have to be paid if the flat is gifted by a Donor?
Yes, Stamp Duty will have to be paid if the flat is gifted by a Donor.