My disk utility won't let me erase hard drive

Just follow the steps, at > What to do before you sell, give away, or trade in your Mac - Apple Support

Try bringing it up and deleting your account as a user. That's what I did with my 2009, 24", iMac.

To add; when you delete yourself as a user, it takes off your data.

You have to start in the Recovery Partition (hold down Command + R on startup) and use disk Utilty there.

You're welcome!