I'm looking to sell my Mac and because I am unable to format and make a fresh reinstall, I'd like to just delete sensitive files.
What files or folders should I delete to be as safe as possible from leaving personal data on the disc?
Everything. Delete all of your applications. Create a new user account with a generic name and password (like "guest"/"guest") and make that account an administrator. Log in with that account and then delete the home folder of your account. These two steps are the best way to get rid of the data. That being said, it is possible to recover this data if the person you are selling it to has malicious intent. Be careful with your data! Best way to be safe is to sell the computer without the hard drive (but that doesn't seem to be an option for what you want to do).
Yes, you should delete all your info. As stated on https://macback.co.uk You should delete all of your personal information off of your device before you sell it. You also need de-authorise iTunes and any other apps that you have given permission to. You’ll want to sign out of iTunes, iCloud, and iMessage as well.