My answers are as follows:
1) After paying bank dues will collect original agreement copy for new registration, now who should possess the original old agreement buyer or seller? If new buyer then when I should hand over to him.
Ans: You may even transfer the loan to the buyer and get the balance consideration amount.
2) When I should handover all the original documents to buyer after registration or after realisation of final payment.
Ans: After getting NOC from your bank, you can get originals from bank and those originals may be handed over to your buyer.
3) At what circumstances I should hand over the flat and when, if customer wants to pay final payment by personal cheque savings not from bank loan. As buyer demanding to handover the flat after 2/3 days of final payment.
Ans: Only after getting NOC from your banker you must hand over the possession to your buyer.
4) Buyer was trying to offer me some 10% cash payment which completely denied
Ans: If the conduct of buyer is suspicious better to avoid sell the property to him.
5) What precautions I should follow to avoid mistakes in such dealings.
Ans: You must be very careful that cheques issued by him are cleared and bank issued NOC to you in respect of your loan.