We built UsedAlert to help people find used deals, so take this in the spirit intended: some things should not be bought secondhand at any discount. Not because used is gross, but because in these categories the risk is invisible, the savings are small, or both.
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1. Helmets
A bike or motorcycle helmet is a one-crash device, and crash damage doesn't have to be visible. The foam that does the protecting can be compromised with no mark on the shell, and the seller may honestly not know. You cannot inspect your way out of this one. Helmets are also one of the cheaper items on this list new, so the used discount buys you an unknown history on the exact product whose entire job is your skull.
2. Car seats
Same physics as helmets, plus regulation. Car seats expire (the plastic degrades), models get recalled, and a seat that's been through any collision is done even if it looks perfect. Secondhand you usually can't verify age, crash history, or recall status. This is the category where "buy new" is closest to an absolute rule; if budget is the issue, buy a simpler new seat rather than a fancier used one. Every seat sold new in the US passes the same crash standard.
3. Mattresses
Hygiene is the obvious problem and not even the biggest one. A mattress wears to its owner's body over years, warranties are void for second owners, and transporting one is how bed bugs commute. The used discount on mattresses looks large precisely because nobody wants them. There's a reason.
4. Earbuds
In-ear buds are personal items with batteries you can't replace. The battery is the part that dies first, it's been aging since the day they were made, and you're putting them in your ears. If you want the discount, this is what Amazon's Renewed program is actually for: Renewed earbuds are inspected, cleaned, and tested, and they're backed by a 90-day replace-or-refund guarantee. Confirm the included tips and accessories on the listing before buying. Over-ear headphones, by contrast, are a fine used buy: pads are replaceable and nothing goes in your ear.
5. Electric razors and groomers
Same logic as earbuds with more skin contact. Sealed batteries, hygiene you can't verify, and heads whose replacement cost eats the discount. Renewed or new.
The pattern
Skip used where the risk is invisible (helmets, car seats), where the item conforms to its owner (mattresses), or where a sealed battery plus skin contact is the whole product (earbuds, groomers). Almost everything else, from cast iron to office chairs to power tools, rewards buying used, and the rest of our used picks are about doing exactly that with your eyes open.