Parities on Narkasa buy-to-sell boards are Cryptocurrency/TRY or Cryptocurrency /USDT. Therefore, you can buy cryptocurrency if you have TRY or USDT in your wallet.
You can convert your existing cryptocurrency into a TRY or USDT cryptocurrency by performing a sales transaction, and you can get a different cryptocurrency.
Tip For instance, you have XRP in your balance/wallet yet you want to buy ETH. For that, you have to sell your XRP assets in the XRP/TRY parity or XRP/USD parity. Assuming that, you made a sale in XRP/TRY parity. With your TRY assets which you obtain from that sale, you can buy ETH in ETH/TRY parity.