There are two types of auctions on OpenSea:
1) English auctions: where the highest bid wins at the end
2) Dutch auctions, where the price falls until someone purchases.
Let's take a closer look so you can make the best choice to sell your NFT on OpenSea.
English auctions (highest bid wins)
OpenSea will automatically complete the transaction if the auction finishes above 1 ETH, with OpenSea paying the gas fee in the process. If the auction finishes below 1 ETH, it's up to the seller to accept the highest offer. The seller is under no obligation to complete the transaction.
Making a bid during an English auction is like making an offer on a fixed-price listing. The seller can choose to accept it at any time, and you'll need WETH - just as you would if you were making an offer on a fixed-price listing.
Please note: Any bids made in the last 10 minutes of an auction will extend each auction by 10 more minutes. Popular auctions are often extended by new bids offered in the last 10 minutes.
You may cancel an auction at any time, but doing so will require paying a gas fee.
Dutch auctions (price falls until someone purchases)
Dutch auctions are similar to fixed-price listings, but the price falls over time. You can buy at any time in the listed currency or make an offer.
Auctions can't have a reserve price lower than 1 ETH because OpenSea covers the gas when the auction ends successfully.
The seller can always choose to accept a bid below the reserve at any time during or after the auction, but the seller will have to pay gas to do so.
For more information on who pays gas fees and when, check out this article!