Refreshing system for sale

Hello, I have a Librebooted Thinkpad - Taurinus X200.
It was purchased in 2019 with Trisquel already installed.
I am looking to sell the machine. I'm not very knowledgeable/ capable with Trisquel, at all.

I could sell it as is and just send a piece of paper with the current username/password, but it has a few personal files on it.
If I just delete these personal files would that be sufficient that no one can ever find them? (if deleted, are they recoverable by the next owner?).

Or should I be reinstalling Trisquel altogether (I'd rather not mess with that if at all possible).
Or perhaps can I remove my current user/ erase it?

What do you think?
Thank you in advance! :)