Will a buyer be able to recover my files if I format my mac HD and sell it?
The accepted procedure these days is that you already have FileVault enabled, right from the first time you use the Mac.

If you do, then the simple format &amp; reinstall in Apple's instructions is totally adequate to prevent any recovery.

If you don't use FileVault, then there is no proven * method to make your data absolutely * irrecoverable.

Therefore, if you don't already use FileVault, enable it first across the entire drive. Wait until it finishes encrypting your data [may be days], then follow the simple format/install instructions.
This is not as quite * as secure as if the drive had always been encrypted, but as good as any "secure erase" procedure.
For practical purposes SSDs are safer * than HDs.

*words in italics would take 20 pages to fully explain.