Executing the sale deed at the actual value of the transaction and paying the differential amount of stamp duty in cash may require explanation of the source of the cash. Accepting the money online would still require explanation of the nature of the transaction
According to the Income Tax Act, this comes under capital gains. The short-term capital gains tax is applied to the profit if the property is sold within two years of purchase while if the asset is sold after two years, then it attracts LTCG.