It is not free to interact with the blockchain. The concept referred to as gas is what the Binance Smart Chain (BSC) and Ethereum have. Gas is the computational "fuel" measure needed in interacting with the blockchain. Each interaction needs gas in a certain amount, depending on the type and number of complicated computations and the required storage. Since NFT smart contracts tend to be highly computationally intensive, simple fund transfers can mean more gas costs.
A gas fee is involved whenever you transfer the sale's net funds to the seller's wallet. Based on the resale's initial contract, the creator will receive a reduced royalty fee because of another gas fee.