What should I check when buying a second-hand power tool?

The seven-minute inspection that separates a bargain from someone else's problem, and the two categories where used is a bad idea.

Difficulty: beginner
Time: 20 min
Read: 4 min
Safety: caution

Short answer

Run it. Listen for bearing rumble and watch for sparking at the vents, check the chuck runs true, flex the lead along its whole length, and look at the ports and vents for the dust that says it was never cleaned. Buy bare tools used and buy batteries new — a used lithium pack is the one part you cannot assess.

Power tools are unusually good used buys. Motors and castings do not wear out from age, most domestic tools have done a tiny fraction of their design life, and the depreciation on the first owner is severe. The risks are concentrated in a few places and they are all findable in a few minutes if you know where to look.

Safety

Never buy a mains tool you cannot switch on, and never buy one with a damaged lead, a cracked case or a missing guard. A second-hand lithium battery of unknown history is the single riskiest thing on any used tool stall — a pack that has been dropped, over-discharged or got wet can fail thermally, and you have no way to see it.

Step by step

Insist on running it.A tool you cannot power up is scrap priced as a tool. Take an extension lead, or for cordless, ask for it to be shown running on a charged pack. A seller who will not demonstrate has a reason.
Listen with the tool at no load.It should spin up smoothly and settle to a steady note. Rumbling or grinding is bearings. An intermittent note or a change of pitch when you tilt it is a failing armature or worn brushes.
Watch the vents while it runs.A little blue arcing at the brushes is normal on a brushed motor. A shower of orange sparks, or visible flame, is not — it means worn brushes or a damaged commutator.
Check the chuck for runout.Fit a long, straight bit and run the tool. The tip should stay put. A bit that describes a visible circle means a bent spindle, a damaged chuck or worn bearings, and it will never drill accurately.
Work the whole length of the lead.Flex it hand over hand from plug to tool, feeling for stiff spots and looking for tape, and check the strain reliefs at both ends. A poor lead is cheap to replace but tells you how the tool was treated.
Test the switch and the speed control properly.Squeeze the trigger slowly through its travel. It should come up smoothly with no dead spots or surging. Work the forward-reverse and the lock-on. Switches fail more often than motors and are often the whole reason it is for sale.
Look at the dust.Open the vents and look. Compacted dust inside a tool means it was used hard and never blown out, which is the single best predictor of a short remaining life.
Check what it comes with, and price the gaps.Chuck keys, side handles, depth rods, guards, cases and, for SDS, the holder itself. Missing parts on discontinued models can be unobtainable, which is how a bargain becomes an ornament.
Check the tool is not stolen.Ask where it came from and whether the seller has the receipt. Site tools are stolen in volume and often re-sold quickly, and a suspiciously cheap set of matching trade tools with the markings ground off is exactly what it looks like.

Tips

Corded tools are the best used value of all, because everyone wants cordless. A corded SDS or a corded circular saw at half price is a genuinely good buy.
Buy a used bare tool into a battery platform you already own. That is where the money is saved with none of the battery risk.
Older trade tools from the big brands often have spare parts still available years later. Check for parts availability before buying anything unusual.

Checklist

It runs, under its own power, in front of you
No rumble, grinding or change of note at no load
No shower of sparks from the vents
Chuck runs true with a long bit fitted
Lead flexed end to end, strain reliefs intact
Trigger smooth through its travel, no dead spots
Vents and ports not packed with dust
Guards, handles, keys and depth rods present
Case and housing free of cracks
Battery, if any, priced as worthless

Questions people ask

Is a used cordless tool worth buying?

The bare tool, yes. The batteries, no — treat any included pack as a free bonus that may be dead, and price the tool as if you will buy new packs.

How do I know if a used tool has been used on site?

Paint and plaster splatter, worn markings, a replaced lead and packed vents. Site use is not disqualifying if the tool is a trade model, since those are built for it, but it should be priced accordingly.

Should I get a used tool PAT tested?

For a mains tool, a check by someone competent is a reasonable precaution and cheap. At the very least, look at the plug, the lead and the case yourself before it goes near a socket.