Friend Suggested Software To Remove Old FIles From Computers

I was talking to a friend of mine about selling all these outdated computers I have taking up space in my house.  They were hand me downs or ones that died and I never took the time to repair them.  We could almost open up a used computer shop with all the computers we have here, it is getting out of hand.  One of my other friends suggested that we read the review of ParetoLogic Privacy Controls and try out their software.  He said that even though you delete most of your files, does not mean they are necessarily deleted off your computers hard drive, so we need this software to delete those files before we could even think about selling them.  I took his advice and wiped out everything off the computers.  I just now have to figure out where or who to sell them to.