There are many second hand shops at many places. But it would be better if you sell to someone who sells on street. They might not give u good rate but it'll help them a lot
You can try Dariyaganj market . It’s a hub for selling and buying books . You’ll definitely get shops there . Whole market is best for books .
You can visit Daryaganj Book Market.
You can go to Daryaganj book market to sell your novels there are number of shops who take old books and yes you can also try kamala nagar or can just ask nearby stationeries
You can sell them at ebay but I will suggest contact some book lovers they will buy from you and will not be greedy for profit