Losing a phone does not automatically destroy every passkey. A synced passkey may still be available on another trusted device, while a device-bound passkey may be gone with that device. Check where each passkey lives and keep a separate recovery route before you need it.
Before paying for a used phone, verify that it can be activated for you, is not reported lost or stolen, works with your carrier and has acceptable return or warranty terms. Then add likely immediate costs to the price.
For a U.S. flight, keep a power bank in the cabin, protect it from short circuits and check its watt-hour rating. A battery at or below 100 Wh fits the FAA's general size limit, but an airline may set stricter rules.
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