Use compressed air to remove dust from fans and heatsinks. For CPUs and GPUs, remove old thermal paste with 90%+ isopropyl alcohol and a lint-free cloth.
Run benchmarks like 3DMark for GPUs or UserBenchmark to prove the part works under load. Taking a photo of the "About" screen or a successful benchmark result increases buyer trust.
Note the exact model, age, and whether the original packaging or accessories (like cables for a modular PSU) are included.