You can find existing smart contracts on NFT marketplaces indirectly through blockchain explorers like Etherscan for Ethereum-based NFTs or Avascan for Avalanche-based NFTs https://avascan.info/ On marketplaces, you can usually view the smart contract details of individual NFTs listed for sale. While reading smart contracts might seem intimidating at first, you can focus on understanding the basic information provided, such as the contract's functionality, token details, and any additional features like royalties or token gating.
Additionally, it's a good idea to carefully review the fee structures and terms of service of multiple NFT marketplaces before choosing where to list your NFTs or buy/sell NFTs, as fee structures and conditions can vary between platforms.