The wiki page for this has surprising amount of useful information here
Usually between 2 and 3 passes will almost guarantee the removal of data from the majority of the layers, but for some tools corrupted sectors can cause issues. Most companies will use shredding services there after to ensure there is nothing coming off of those disks, so as for re-selling them i'm not entirely sure how that will work ;).
Also to be sure you will be looking for digital forensics, there are degree programs surrounding this topic so don't expect to learn it in a day. The most common tool used is this one the sleuth kit, there are plenty of blogs around for tips and tricks when using it.
edit dd the unix utility is also awesome for this sort of thing