Used tools are among the best value in the trades and among the easiest ways to waste money. The difference is a short inspection done in the same order every time, before any cash moves.
Before you plug anything in
Housing. Look along every seam for cracks, especially around the gearcase and the handle. Hairline cracks spread. Check that all the fasteners are present and that the heads are not rounded from repeated disassembly.
Guards and safety features. Everything that should be there needs to be there and needs to move as intended. A missing guard is either a safety part to source or a walk-away.
Cord and plug, on corded tools. Run the cord through your hand end to end. Look for cuts, taped repairs, and stiffness. Check the strain relief where the cord enters the tool, which is where damage hides. Any missing ground pin is a no.
Battery interface, on cordless tools. Inspect the rails and contacts for corrosion, melting, or discoloration. Heat marks at the terminals suggest a hard life or a bad connection.
Vents. Packed with dust means it ran hot for a long time. That is a maintenance signal, not automatically a dealbreaker, but it changes what you offer.
Chuck, collet, or anvil. Open and close it. It should move smoothly and grip concentrically. Look at the jaws for wear.
Smell. Put your nose near the vents. A burnt electrical smell is a walk-away. Motor windings that have overheated do not come back.
Water. Look for rust inside the vents, mineral staining, and any silt line. A tool that has been submerged is unpredictable even if it currently runs.
Now run it
Start and stop. It should start smoothly, not with a violent jump. Listen for a healthy consistent motor tone.
Bearings. A rumble, growl, or grinding that changes with speed means bearings. Bearing work usually costs more than the tool is worth on the used market.
Vibration. Excess vibration through the housing suggests bearings, a bent shaft, or a damaged fan.
Switch and speed control. Run the trigger through its full travel several times. Variable speed should be smooth and continuous, not steppy or intermittent. Test the forward/reverse and every mode selector.
Under light load. If the seller allows it, put the tool into a scrap piece. Bogging, cutting out, or a change in tone under mild load points to electronics or a weak motor.
Brushes, on brushed tools. Some have external brush caps. If they come out easily, look at the length of the carbon remaining. Brushes are a cheap part and a good negotiation point.
Cordless deals and battery reality
The batteries are the least predictable part of a used cordless purchase. A pack can look perfect and hold a fraction of its original capacity. Age alone degrades cells even in storage.
Assume any included battery is a bonus rather than value, and price accordingly. If a pack is swollen, warm without use, or has a cracked case, do not take it at any price.
Charge and discharge behavior is the only real test, and most sales do not allow time for it. That is why used bare tools from a platform you already own are usually the better used buy.
Questions worth asking
Why are you selling it. How long have you had it. Has it been repaired. Do you have the charger, case, guard, and side handle. Does it come with the original accessories.
The answers matter less than how readily they come. Somebody who knows the tool talks about it easily.
What the price should reflect
A clean tool with light wear, all accessories, and no faults is worth a substantial fraction of new. A tool missing its guard, with a taped cord and packed vents, is worth parts value. Most listings sit in the middle and the honest price sits below what the seller hoped.
Walk away from burnt smells, water damage, cracked housings, bearing noise, and anything the seller will not let you run. There is always another tool.