What should I never buy secondhand?

A short list, and it is really one idea. The page The line in the secondhand aisle puts it in a sentence: almost everything is fine used, and the exceptions are the things whose damage is invisible.

Never a helmet, a child’s car seat, or climbing gear. That page’s safety line, in case you read nothing else: these protect you by breaking. The foam inside crushes to absorb a blow and does not spring back, so one impact you were never told about uses the item up while leaving it looking brand new, and there is no squeeze or flex you can give it on a table that tells you. They also age out on their own, from heat, sunlight and time, and every helmet and car seat carries a date; find that date before you trust one, including one you already own. This is the one group where being wrong is measured in something other than money.

Two more groups, for different reasons. Things that carry passengers: mattresses and upholstered furniture, because stuffing and seams are places you cannot inspect, and bed bugs live in exactly those places. For anyone who has been through an infestation or could not pay to deal with one, that page calls it arithmetic, not caution. And things that sit against skin and do not fully clean, underwear, swimwear, pillows, toothbrushes, breast pumps, where people’s own thresholds legitimately differ. Everything else is a negotiation, not a rule.

What the used market is good at. The same page says the savings are biggest exactly where new prices are worst: books, where the words are identical; solid wood and metal furniture, exercise equipment, tools; kitchen goods, on the condition that you wash things properly before they join the house; older-generation phones and appliances; baby clothes and gear, which are nearly new because babies outgrow things before they wear them out, and which is emphatically not a route into car seats. Cars carry a footnote: for a few very reliable models the gap between used and new can shrink far enough that the warranty and the newer safety systems are worth the difference. And the part people arrive at last: what survives on the secondhand rack has already been through an experiment, so a used tool or chair is a filtered sample, not a degraded one.

Price electronics as if they are broken. Price thrift electronics as if they’re broken handles the shelf where there is rarely an outlet to test and no returns desk. Mentally pay the price of a broken one; if a dead one would sting, walk. Powering on and working are different claims. One person found a two-dollar router that turned out to be an expensive working model; they had priced it as junk, so it was pure upside.

Let someone else open the box. Buy it once: the boots rule supplies the reframe that makes quality affordable: the thing is used the moment anyone opens the box, including you, and well-made things are exactly the ones that survive a first owner, which is why the used market is full of them.

Who this is not for. The secondhand page’s own smaller warning: jigsaw puzzles are a gamble because missing pieces are common, and that is a category where the risk is only that you are annoyed, worth remembering while reading the rest of that page. Everything else on this answer follows from one test: if the damage that would matter is damage you could not see, buy it new.