You said bulletproof, so the only two options I know are: 1) Physical Drive Shredder. You can pay companies to do this for you if you don't know anyone that o

You said bulletproof, so the only two options I know are: 1) Physical Drive Shredder. You can pay companies to do this for you if you don't know anyone that owns one. 2) Kiln. You can melt the drive. Avoid inhaling the fumes. Some regions may consider this a violation of environmental laws, so look that one up first. Any form of software wiping or changing encryption keys leaves doubt. Data, blocks, keys could in theory be backed up in protected sectors that require vendor tools to view or change. The above are just to answer the question of "bulletproof". They are not really the answer in your case however. You are selling the computer, so just take the drive out of it and put in another used one. You can find used drives online. Or just say, "No HD included"