In order to pay for transactions on OpenSea, Rarible, or any other Ethereum mainnet NFT trading platform, you need to have enough ETH in your wallet to pay for gas. That said, there are certain circumstances where a platform, such as OpenSea, will require you to pay in WETH, which stands for "Wrapped ETH". It is a different form of ETH, created to be more compatible with newer smart contract functionality. Please consult OpenSea's documentation here for more information on paying gas fees on the platform.
For more information on how to send and receive NFTs, see here.
For more general information on how the Ethereum network operates, see here.