1. The agreement with the developer that you enter should have a clear provision that the Sale Deed would be executed by the developer in your name or in the name of your nominee (i.e. in whose favour you would like the sale deed to be executed by the developer.). If this clause is there, then as and when the Sale Deed is to be executed by the developer, you can ask / direct the developer to execute the Sale Deed straightaway in the name of the person to whom you would be selling the flat. In this way, you would be saving on the expenses towards Stamp Duty (i.e. first from the developer to you and then from you to the buyer.)
2. But your responsibility under the contract towards your buyer to execute the Sale Deed of the flat in his favour would remain. It is you who would be responsible for giving the constructed flat to your buyer AND NOT THE original developer. In case the original developer fails to hand over the flat and fails to execute the Sale Deed for whatsoever reason, then you would be liable to pay damages etc., to your buyer.
3. You may be able to take action separately against your developer for the breach of contract.
4. While there will be a service tax implication in respect of your agreement with the developer, no such liability will be attracted in respect of the sale of the under construction flat by you to your buyer.