Selling some old hard drives on ebay... is writing zeros good enough?

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I still have to see any credible proof that data can be recovered (using just software) once overwritten. If that were the case, hard drive makes would use such software to basically double the capacity of their drives. Of course, maybe if you open the patters and use something like Magnetic force microscope, or similar tools, then maybe you can say with some probability what was written on the platter before the latest data was written, but who would go to such trouble (not to mention who would have access to such tools) with a drive they bought on eBay?

if someone has a link to a good article that explains how the overwritten data can be recovered using only software, please share the link. Thank