It is safer to erase using the Zero-all-blocks option. Otherwise, the buyer might be able to retrieve some of your files using disk-repair utilities.
If you do zero it out, be sure to plug into AC power first, because that type of erase will take a really long time.
I recommend this type of erase (or format) before a sale. The time required is preferable to their being able to retrieve your CC number, or SSN, or your library of revealing gf pictures.