Before wiping, back up what you need, then sign out and deauthorize anything tied to the machine so it cannot reactivate later. That includes iCloud and Find My on Macs, your Microsoft account on Windows, plus Office, Adobe, Steam, and any 2FA or password manager apps. Remove the device from your Apple ID or Microsoft account device list so messages and activation will not follow the new owner.
On Windows, use the built in Reset this PC with the secure option. On Windows 10 go to Settings > Update & Security > Recovery > Reset this PC, and on Windows 11 go to Settings > System > Recovery > Reset this PC. Choose Remove everything, then select Change settings and turn on Clean data so it fully cleans the drive. If your edition supports it, enable BitLocker first and let it fully encrypt, then do the reset so discarding the key renders old data unrecoverable. On a Mac, sign out of iCloud and turn off Find My, then boot to Recovery, open Disk Utility, erase the internal drive as APFS, reinstall macOS, and power off at the Hello screen. For Linux or older systems, use the drive maker's Secure Erase from UEFI or a utility like Samsung Magician or nvme format for SSDs. For hard drives, a single pass zero fill is sufficient.
After the wipe, boot once to confirm it only shows the initial setup screen and no user accounts or files. In BIOS or UEFI, load defaults and clear the TPM on a Windows PC so no old keys remain. Clean the laptop, note exact specs and battery health, include the charger, and you are ready to list it. If you are extremely cautious or the drive is failing, pull the drive and sell the laptop without storage.