When you delete files, the first letter of the file name is replaced with a dollar sign. In programming technology, this effectively makes it "non-existent" In that stage, it is easily recovered with a hex editor. It will not be using any resources, including memory, and merely exists as someelectonic "marks" on the Hard disk.
If you do a quick format, the hard disk is rearranged into sectors. This will scramble some of the file for ever, but only a few. In a hex editor, again, it is more difficult to spot the individual files.
With a full format, the whole hard disk is overwritten. A retail shop would not, normally, have the facility or interest in trying to recover data, at that stage.