Right now, Luno isn't an option for selling your bitcoin anymore except if you want to sell and don't want to withdraw your fiat but if no, you can try using Binance or another exchanger.There are 3 ways through which you can sell your Bitcoin, you can sell straightforwardly to your friends or families who need to purchase Bitcoin, or through a P2P crypto trade or through a crypto exchange
I personally sell my Bitcoin through a crypto exchange which I think is the best and most secure option.
I prefer to use Luno and coinbase platform mostly because they are one of the Best to use in my country, Trustworthy, Easy UI and they are looking to add more coins in future.