Selling flat

This is a serious issue. People are getting better and better in making fake currency notes. It is very difficult to detect unless you have a proper instrument such a UV light. Then again, if the amount of cash is large, are you going to check the notes one by one?

its better u involve a broker for the transaction for a fair dealing. ur money will be secure plus the tension of black and white money will be his responsibility.

Better to opt for cheque or online account trfr if u doubt so much.

coz even after detecting some notes looks so real  that you can recognise that it was not real. 

It would be advisable to tell the buyer to initial his signature on every note. If fake notes are found out later,they can be returned to him.There is no other unless you know some in bank who will check notes on their machine for you.