deleting data

You should be able to create a new admin user account, then log out of the original admin user account and into the newly created admin user account. Then delete the original user account(s).

Set up users, guests, and groups on Mac - Apple Support

Delete a user or group on Mac - Apple Support

When receiving a pre-owned computer you should always perform a clean install of the OS by first erasing the whole physical drive before installing the OS. You never know what nasty surprises you may find that has been left behind by the previous owner (malware, spyware, corrupted/broken OS, etc.). Plus you may not have a legal right to some of the third party apps installed by the previous owner.

For macOS 10.6 to 10.10 use the instructions in this article:

https://www.owcdigital.com/assets/support/support-formatting-and-migration/Mac_Formatting_6-10.pdf

For macOS 10.11+, the procedure is different because Apple has changed how the "partition" tab works in Disk Utility with later versions of macOS.