Used or new: how to think it through, one category at a time

The discount on a used item is genuine, and it is also a payment. You are being paid to take on some risk, to give up some recourse, to accept a shorter remaining life, and to spend your own afternoon on collection and inspection. Sometimes that is a wonderful trade. Sometimes it is a poor one dressed as a bargain, and the difference has almost nothing to do with the size of the discount.

Four properties of the object decide it. How fast it loses value compared with how fast it wears out, whether you can inspect the part that fails, what happens if it turns out to be broken, and how long anyone will keep supporting it. Run those four and the answer usually falls out before you have opened a listing.

Two curves that are not the same curve

Everything has a depreciation curve and a wear curve, and used buying is good exactly where they diverge. A solid wooden table loses a large share of its price the moment it leaves the shop and loses almost none of its usefulness for decades. Hand tools, cast iron, most furniture, garden equipment, musical instruments and good cookware behave the same way. You buy a small amount of wear at a large discount, which is the entire case for second-hand.

Then there are the things where the two curves run together. Anything with a battery in it is quietly burning through its life whether or not anybody uses it. Anything with a consumable core is closer to the end than its appearance suggests. There the discount is roughly honest, the saving is smaller than the price difference implies, and the case for used gets thin fast.

Can you inspect the thing that actually fails?

This is the question that separates a good used purchase from a gamble. With a table, everything that can go wrong is visible: joints, warping, water marks, a wobble. With a sealed compressor, a corroded connector under a dashboard, or a board that met a glass of water eighteen months ago, none of it is visible, and the seller may not know either.

The rule of thumb: the more of an item's value sits in a component you cannot see or test in ten minutes, the larger the discount needs to be, or the more you need a professional inspection, a documented service history, or a warranty from a business. If none of those are available and the hidden part is most of the value, you are not buying a discount. You are buying a lottery ticket with a delivery date.

• Run it through a complete cycle, not just an on switch. Things fail at the end of a cycle, under load, or when hot.
• Look at the wear parts: belts, seals, hinges, tires, cables, brake pads. They tell you how the whole object was treated.
• Check corrosion where it hides rather than where it shows, which usually means underneath and behind.
• Find the serial or date label and compare it against what the seller told you about age.
• Ask what was replaced and when. A seller who can answer that has usually maintained the thing.

What happens if it turns out to be broken

The most underrated thing new money buys is recourse. In most places, buying from a business carries consumer protections that a private sale between two individuals simply does not, and the specifics vary enormously from country to country, so it is worth knowing what applies where you live before you assume either way.

Then there is practical recourse, which is often the one that matters. Can you find this seller again in a week. Is there a record of the transaction beyond a message thread. If the answer is no, then the honest way to think about the purchase is that it is final at the moment of handover, and to price it accordingly.

The clock you cannot see

Anything with software in it has an expiry date that has nothing to do with its physical condition. When updates stop, a perfectly functional device gradually stops working with the apps, accessories and services around it, and in some cases stops being safe to put on a network. That date is set by the manufacturer, it is usually published, and it is measured from the model's release rather than from your purchase. A device three years into its support window costs less for a reason.

The mechanical equivalent is parts and expertise. Before buying an older appliance, machine or vehicle, spend five minutes checking whether the parts that commonly fail are still available and whether a local repairer will still work on it. A cheap machine nobody can source a seal for is not a cheap machine.

Where used is usually the wrong answer

• Safety equipment whose history you cannot verify. A child's car seat or a helmet that has absorbed an impact can look untouched and no longer do its job.
• Anything where the value rests entirely on an unverifiable claim, whether that is authenticity, a service history or a repair somebody says was done.
• Items tied to a proprietary consumable that has been discontinued, where the purchase price is the smallest part of what you will spend.
• Anything at the very end of a maintenance cycle, where the first service after you buy it costs as much as the item did.

Deciding in five minutes

1. Write down the real difference in price, not the headline one. Add collection, delivery, immediate servicing, replacement consumables and anything missing from the box.
2. Compare expected life rather than price. An item with half the remaining life at half the price is not a saving; it is the same deal with more inconvenience.
3. Name the failure mode. If you cannot say what is most likely to break, you do not know what you are inspecting for.
4. Price being wrong. If the thing dies in a month, is that annoying or is it a genuine problem? Cheap and replaceable can absorb far more risk than expensive and essential.
5. If the discount is modest and the risks are hidden, buy new. If the discount is large and everything that fails is visible, buy used with a clear conscience.

There is also a middle path worth knowing: refurbished or ex-display stock sold by a business, which keeps some of the discount and most of the recourse. Check who stands behind the warranty, though, because a seller's own thirty-day promise and a manufacturer's remaining coverage are very different pieces of paper.

The pattern that emerges after a few of these is easy to state. The best second-hand purchases are boring objects with nothing hidden inside them, bought from someone who can tell you how they were used.