Well, there is a way to restore the files even it was already deleted by just using a powerful recovery software. Although there are software's that claims a 100% crumb free file shredder. I think there are still files that can be recovered via software. If you want to know a good safe file deletion software, you can check this site for your reference:
https://www.lifewire.com/free-file-shredder-software-programs-2619149
But if you really think that the files on the HDD is really important, i suggest that you replace the HDD first before selling it. Most laptops HDD compartment are easy to open, some of them have only two screws securing it. Just unscrew, open the compartment and pull the HDD. If your not familiar on how to remove the HDD you might want to get some help from a friend. What to do with the old HDD? i suggest you destroy it so that you can be assured that your data is safe. I hope this helps.