If this is your first listing with OpenSea, sellers must pay two transaction gas fees to setup their account. This is a one time fee that you will not pay moving forward.
Sellers do not have to pay gas fees to "mint" each individual item until the item is sold. This is referenced as lazy minting, the NFT is minted upon sale vs minted upon listing.
Sellers & Buyers
There are gas fees when the item is sold and who pays the fees, depending on transaction.
Buyers pay the gas fees when purchasing fixed-price items.
Sellers pay the gas when accepting offers.
For more information on when OpenSea users pay gas fees, please click here.