Selling NFT’s through smart contracts vs NFT marketplaces

Guys, this may be a stupid question so I apologize in advance. I own a rare NFT and someone wants to buy it via a smart contract vs just offering it on Opensea because "gas fees". Is this legit or is this scammish? A lot of potential money is at stake here and I don't even know if this is legit or not. They sent me the smart contract code as well as the smart contract address. Happy to answer any more questions without giving away too much information. Thanks!