Start with the seller’s setup
A used tower is usually easier to check than a laptop because you can see most of what is inside. Ask the seller to meet where you can plug the machine in, and ask them not to start it before you arrive. A cold start tells you more than a machine that has already been warmed up, since some faults only show when parts are cold.
Bring a small flashlight and, if you are comfortable using one, a screwdriver that fits the case. Ask permission before you open anything. A seller who refuses to let you look inside a desktop is giving you information too.
The outside of the case
Walk around the case slowly. Dents and scratches are cosmetic, but pay attention to what they suggest about how it was treated. A badly dented corner may mean a drop, which can loosen parts inside. Check the front panel ports and buttons, since these take daily use. Plug a flash drive into each front port and try the power button several times.
Look at the vents on the front, top and back. Heavy dust, pet hair or a sticky film over the grilles points to years of poor airflow, and the article on case airflow explains why that matters for temperatures. Look for rust, water stains or a chalky residue on metal parts, which can suggest moisture or a spill.
Smell and sound
Smell is a surprisingly useful test. Cigarette smoke is a cosmetic issue but is hard to remove. A sharp, hot or fishy smell can be a sign that a part has overheated or failed, and a burnt electrical smell is a reason to walk away. Musty odors may point to damp storage. A little dust smell on a warm machine is normal.
Once the machine is running, listen. Fans typically produce a steady whoosh. Grinding, rattling or a rhythmic ticking can mean a worn fan or a struggling hard drive. A fan that screams up to full speed shortly after startup might be dealing with blocked airflow or poor cooling.
Open the case
Switch the computer off and unplug it before opening anything. Then remove the side panel according to the case design. Look for the following:
- Dust and debris. Some dust is normal. Thick, matted dust on heatsinks means the cooler has not been cleaned, and you will want to do some cleaning of the computer after purchase.
- Loose parts. Gently check that cards, cables and drives are seated firmly. A loose screw rattling around the bottom of the case is worth noting.
- Burn marks or bulging. Look at the motherboard and around the power connectors for dark patches or discoloration. Round capacitors with domed or leaking tops are a bad sign.
- Evidence of repairs. Sloppy cable routing, mismatched screws or scratched screw heads suggest the case has been opened often. That is not necessarily bad, but ask why.
Check that the parts make sense
Mismatched components can tell a story. A very old processor with a brand-new expensive graphics card may be fine, but it might also mean the machine was patched together from spare parts. Compare what you see with what the listing claims. Memory modules in a pair should typically match; the article on memory modules and compatibility explains why. A computer with an empty memory slot and only one module may simply have room to grow.
Look at the power supply label and the cooler. A tiny power supply feeding a power-hungry graphics card would be a concern, and a stock-looking cooler on a heavily advertised high-end processor might mean parts were swapped. If the seller cannot say what was changed and why, treat the listing with caution.
Find the storage and check it
Locate the drives and confirm what they are. Solid-state drives are small and flat and have no moving parts, while hard drives are larger and make audible noise. Ask the seller whether the drive has been replaced. If the system allows it, open a drive health report. Anything that suggests the drive is already failing is a good reason to renegotiate or leave. A computer whose storage is close to the end of its life is a poor value, however cheap the rest looks.
Check that the listed capacity roughly matches what the system reports. Some gap is normal, as the article on advertised capacity explains.
Run the machine for a while
Leave it on for ten to fifteen minutes. Open several programs, play a video and, if you can, run something demanding. Notice whether the case becomes uncomfortably hot, whether the fans change speed rapidly or whether the computer restarts unexpectedly. Sudden restarts often point to heat or power problems, and the random restarts article describes the usual causes.
Also test the outputs. Connect a monitor and check that each video connector the seller mentions actually works. Try the audio jacks, USB ports and network port. Look for a clear picture and no flickering.
Do not skip the boring checks
Ask for the power cable and manuals, and photograph the serial number for your receipt. Ask the seller to sign out of any accounts and clear any firmware passwords in front of you, because a locked machine can be unusable.
Know when to walk away
You do not need to find a perfect machine. Dust, scratches and an old operating system are normal, and they are fixable. Burn smells, a swollen or leaking component, a seller who will not let you test the machine or any sign that the computer was not originally what they describe are different. If your gut says something is off, thank the seller and leave. Another listing will turn up, and an unchecked purchase is rarely a bargain.