OpenSea is decentralized, so we never hide or even escrow your funds. As a result, if this is your first time making an offer, you need to complete these free (plus gas) transactions:

After those, you'll see a signature request to finalize your offer. For future offers, you won't have any transactions at all, only a signature!

Note that you can always access your ETH & WETH balance and view/cancel your active offers from your account page.