I have a Solidity Smart Contract, I'm need to sell/return to the smart contract owner the NFT tokens that users bought from the ERC721 so that I can resell them.
How can I do that?
If I understand right, you're asking for a way to sell ERC-721 tokens.
In general, you can sell any tokens if someone is interested in paying for them. There exist various on-chain NFT marketplaces for trading NFTs.
But, if you consider this from the other perspective, you can't sell NFTs if nobody wants to pay for them.