The short answer
Prioritize a clear return policy, battery standard, warranty, unlock status, and exact model over cosmetic grading alone.
Step by step
Buy from a seller that states the warranty, return window, testing process, and battery-health standard in writing.
Verify the exact model number, storage, network compatibility, unlock status, and eSIM or physical-SIM support.
Check that the device is not activation-locked and ask how the seller screens lost, stolen, or financed phones.
On arrival, inspect cameras, speakers, microphones, charging, buttons, biometrics, display, battery, and cellular service.
Complete all tests before the return period ends and keep the listing, receipt, and serial number.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.