Can i sell the booked property in delhi ? is it legal ?

In my opinion, it should be perfectly legal. If you want to sell your booked plot in Delhi to someone, you can draw up a new agreement with the help of any local advocate (or even the property dealer may assist you,) with the person to whom you may want to sell.

In return for payment by new purchaser to you (plot market price- asauming you paid the whole booking amount, plus some interest that you may charge if the booking was done by you months ago,) all the documents and receipts that you recieved for the payment you made in the course of such booking shall be transferred to the new purchaser.

Please note, the rules for private builders and government housing schemes may differ. Anyhow, it will be clearly stated in your agreement whether or not/how you can transfer your booked plot to someone else. Again, I'd suggest you to get in touch with an advocate so he or she can verify if it's possible and make the process easier.