You can make an offer on a single Solana item or on a whole collection. Your offer amount is held with the offer onchain, so it leaves your spendable balance until the offer ends.
Open the item and click Make offer, or open the collection and click Make collection offer.
Set your amount and how long the offer lasts, then approve the transaction in your wallet.
Cancel the offer at any time from your profile, and your funds return to your wallet. Offers are canceled one at a time on Solana, and each cancelation is an onchain transaction you sign and pay the network fee for.
When an offer you made using OpenSea expires, your funds return to your wallet automatically, shortly after the expiry time.
If the owner accepts, the item transfers to you and your offer amount is paid out. The OpenSea fee and any creator earnings come out of that amount, and the seller receives the rest. You pay exactly the amount you offered, with nothing charged on top.
The currency is set by the collection, not chosen by you. Most Solana collections are priced in SOL, so offers there are made in SOL. A few are priced in USDC on Solana, and offers there are made in USDC. You cannot offer SOL on a USDC collection or the other way around.
A few more things worth knowing:
Because your funds are held, you cannot use the same SOL for several offers at once.
Making a new offer on an item you already have an offer on replaces your existing offer rather than adding a second one.
If you own an item that is listed and you accept an offer on it, you do not need to cancel your listing first. OpenSea includes the cancelation in the same flow, and you may be asked to sign two transactions.
Collection offers are not available on very large collections, or on collections whose items are grouped under more than one onchain collection address. A collection offer also only applies to items in the grouping it was made against.
Trait offers are not currently enabled on Solana collections.
Offers you made on another Solana marketplace are shown on OpenSea but cannot be canceled here. Cancel those where you made them.