OpenSea's auctioning system minimizes the number of on-chain Ethereum transactions required to list an item for sale. In fact, once you've completed the preliminary steps, auctioning an item will require no on-chain transaction at all. Here's how it works:
1. For your very first OpenSea auction, you will need to initialize your account with a free (plus gas) on-chain transaction. You only need to do this once!
2. When you auction a new "type" of item (for example, your first CryptoKitty or your first Autoglyph), you will need to complete another free (plus gas) transaction in order to unlock access to that item type. In most cases, you only need to do this once per item type! So, for example, once you've approved access to Autoglyphs, you can auction as many Autoglyphs as you like without having to complete another transaction.
3. Once you've unlocked access to that item type, all auctions will simply require signing a message with your wallet. No gas required!
While there are a few transactions up front, the overall result is a much cheaper, faster experience for the end user.