Network fee — is a payment for processing transactions. To buy/sell an NFT, you need to have a minimum of 0.3 GRAM on your balance: it's requested by the network's smart contract, and without this gas the transaction won't ride anywhere.

What else: upfront fees and returns
You might wonder: "Do I need to pay 0.3 GRAM on every transaction?"

No, not the whole amount

Due to the network architecture, it asks for a bit extra upfront so that the transaction is guaranteed not to get stuck.

As soon as the operation goes through, the blockchain automatically returns most of the amount back to your wallet. The actual fee ends up being just pennies.