In India, we have 'The Antiquities and Art Treasures Act, 1972 which is applicable to the entire nation. In this antiquity includes any coin which has been in existence for not less than one hundred years.
Under this, if one intends to sell any article that comes under the preview of this act, then he/she needs to apply for a licence to do so. At times, based on the value (not monetary) of the particular antique item, the Government may decide to acquire it, register it and conserve it.
I would suggest, if you are not a numismatic expert, please meet the experts of such numismatic societies that are active in major cities. Here you would get an idea of how valuable your article is and also how to go about selling it to other countries without unknowingly breaking the law. This link has the list of major coin clubs in various cities and states (http://goldencollection.co.in/list-of-numismatic-society-club-of-india-in-2019/)
The other option is to approach various well known Philately, Arts and Coin trade establishments that can sell the article on your behalf for a certain commission percentage.
If there is any debate about how valuable the coin is then you can approach the Director-General of Archaeological Survey of India and an officer authorised by him/her. Any individual is legally not allowed to export any antique item unless he/she is authorised by the Government of India to do it on it's behlaf. (please refer page 3 and 4 of the pdf below)
Please read this pdf that covers antique items and the Indian rules. (https://www.indiaculture.nic.in/sites/default/files/Legislations/8.pdf)
Once you have the appropriate valid license then you can sell it. Please ensure that you insure the object adequate and yes, you would need to pay tax on the earnings (profits) from the sale. Before shipping any antique overseas, please discuss with the courier company or the International speed post division.