Are bids an assistant places binding?

Yes. A bid is binding however it's placed. If a seller accepts it, the card on your account is charged automatically and the box ships to your default address — there's no checkout step and no second confirmation. If that card is declined, we try your other ways to pay before the bid fails: your bank account first if you're approved to pay by bank, then your other saved cards. You can set that order yourself under Paying for your bids on Account → Bids.

So treat a bidding key like a card in someone else's hand:

• Set the per-box cap to the most you'd be happy to pay, not the most you can afford.
• Use a Look only key unless you actually want the assistant bidding.
• Confirm the product, the price per box and the quantity before you tell an assistant to bid. A good assistant will read the market back to you first.
• Keep the key private. It's a password: anyone holding it can bid as you, up to its cap.
• Revoke a key the moment you stop using it, when it may have been seen by anyone else, or if you're unsure. Revoking is instant, and it doesn't touch bids that are already placed.

Bids placed by an assistant appear under Account → Bids like any other, and you can raise or cancel them there.