It depends. If a seller on OpenSea lists an NFT at a fixed price, the buyer pays the gas fees associated with that transaction. If a seller accepts an offer from a prospective buyer to purchase the NFT, the seller pays the gas fees associated with that transaction. The seller does not pay the gas fees for auctions, unless they manually accept an offer during that auction. When lazy minting, the creator does not have to pay gas fees to create their NFTs.