Such old notes and coins fetch huge prices due to their unique serial number and historical significance. That way, instead of a 1000 rupee note, you will get up to 3 lakh rupees.
If you have a 1000 rupee note with serial number AH17 75, its value is more than 3.5 lakhs. It is a British banknote.
Indian currency notes with serial number 786 are also fetching good prices. Any old currency notes that have this serial number on them will fetch you a lot of money online. It is considered a rare note.
There are many websites like ebay, olx, coinbazzar to buy old currency notes and coins like this. Go there and register yourself as a seller and upload a photo of your rare notes.
The person who wants to buy it will contact you and give the price and buy it. But the price for these old coins and notes depends on market conditions and demand. It is impossible to predict in advance that it will get such amount. It depends on the person buying it.
Similarly, making money by selling coins and currency notes online is entirely at your own risk. Because RBI will not take responsibility in this matter. This is a fully unauthorized transaction. So you can try selling coins and notes online in the name of your own choice.