1. land owner can sell the flat from his share before he receives the possession
2. a tri party agreement will be required between owner, builder and purchaser. The purchaser will make payment to the owner and the builder will agree to give possession of the flat to the purchaser instead of the owner
3. when builder allots flats to owner against grant of development rights by the owner, the owner is taking construction service from the builder. But since the owner is being paid in kind (i.e. in the form of flats) and not by way of money, for grant of development rights to builder, the builder will be liable to pay GST. Since here a purchaser is buying a flat from the land owner and the builder will be allotting the flat to the purchaser, the purchaser will be required to pay GST to owner. The owner will then have to deposit the GST with government treasury
4. The owner can give POA to builder to sell the flat from the owner's share OR the owner and builder can enter into a tri party agreement with purchaser
5. you are right. POA granted to a non relative with a power to sell will attract stamp duty which is applicable to conveyance. The stamp duty paid on the POA can then be adjusted against the stamp duty payable on the sale agreement for purchase of flat from owner's share
6. you will have to make payment to the land owner since you are purchasing a flat from his share