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In the context of demonetization of Rs500 and Rs.1000 currency, the real estate business affected a lot. The sellers who demands the rest of the money over and above the registration values in Black is a common issue. On demonetization of currency the buyers are finding it very difficult to arrange for payment of the Black amount and it is not wise also. It is fair on the part of the sellers of properties to stick to sell the property for not more than the guidance value and it helps in eradicating the Black Money business.

Since you are a Seller, you feel, collecting black money & cheating the government/buyer is fair !!.
If at all a seller want to save the capital gain tax, he/she can always reinvest it real estate or keep it in Tax saving account legally ?

But you haven't answered, why should a buyer, having legal money earned via salary/loan has to pay in unaccounted form to a buyer.
Not sure, by registering the property at Guidance value, how can you eliminate Black Money from the financial Market. ??