Definitely a grey area both from airlines and card issuer terms of service. However there are agents in US who pay you for using your miles and hotel reward points. Not sure about the scene here.
Way it works is these are travel agents who are the buyers and need miles to book business or first class for clients. The need a couple hundred thousand miles for each trip and source the miles from individual sellers (like yourself.) They offer you a rate for your miles and in exchange you give them credentials of your miles account. They then proceed to transfer the miles from your account to theirs in one to seven days depending on what they need at the moment. After they have transferred out the miles, they pay you via PayPal.
I have done a few transactions 4-5 yrs back while I was in the US pretty smooth and hassle free. Should work the same way for India but not sure what is your recourse in case things go south.