Buying used miners: inspection and seller checks

• Hash boards — corrosion, burnt spots, warped fins, non-original solder work = walk away or price a board swap.
• Fans — spin freely, no bearing growl; fans are cheap but failures cook chips.
• Control board & firmware — boots clean, stock firmware available (custom firmware can hide clocks and errors).
• Ask for a live test — a 10-minute run screenshot with hashrate, wall power, and per-board temps beats 50 photos.
• PSU — included? Same inspection; PSUs die with age regardless of care.
• Hours & environment — a farm-retired unit ran hot and dense; a home unit usually ran gentler. Ask where it lived.

• History on the marketplace (account age, reviews, dispute record) — not just a polished listing.
• Escrow or buyer protection; wire transfer to a stranger is a donation.
• Warranty: any transferable coverage? Who pays return shipping if DOA?
• Price sanity: compare against current new price AND expected repair cost of one hash board.
• Serial numbers verifiable with the manufacturer where the brand supports it.

Match the candidate machine to a live network first — a cheap used GPU against a small chain's current difficulty and price (project pages) beats any listing's 'estimated earnings'.