While I agree that the 7 pass method is generally sufficient, and the 35-pass pretty safe, neither are guarantees.
No matter how many times you write over your data, the right person could retrieve it.
The question, is are you going to sell it to that right person, or just some ordinary user who doesn't really care? Or, depending on how paranoid you are, who might that user then sell the computer to later?
Personally, I've done both the 7-pass and the 35-pass methods on the systems I've sold more recently.
But, if you are really concerned, then I agree that physical destruction is the only option. Unfortunately, I've heard of people recovering data from drives that have been through severe destruction methods. So, melting it into a liquid is pretty much your only guarantee. And, strangely, I do seem to recall reading something about a weakness in that method as well (though I cannot recall what the statement was).
Here's the way I see it. If you plan on using the computer for say 3 or 5 years, then the capacity hard drive that it ships with today will be pretty cheap by then. Probably less than $50.
So, assuming it's a computer you can open up and install a replacement drive yourself, then keeping the current drive and replacing it with an unused on at that point would be reasonable. You would spend very little, and know that they didn't have your data. And, it wouldn't cost enough to affect your profit margin much.
Now, if it's a newer iMac. Then, it's not really something you'll want to tackle. Then you're talking about using a shop to replace the drive. And, it introduces risks of what might go wrong and what it might cost if they introduce a problem in the reassembly of the computer.
In that case, I'd go with say wiping the drive with a 35 pass method. And, then repeat that as many times as you want to feel better.
Odds are, no one will care enough to look. But, then someone might. And, it's only the ones with the bad intentions that are likely to care enough to look.
I used to service computers. And, I'd stumble on stuff. But, I wouldn't use data recovery tools to go looking (unless specifically asked to). But, when someone came to me and asked me to find problems or find what was causing problems, I'd browse everywhere and report to them what I found.
I'd often find some embarrassing stuff. And, I'd let them know what I found if I did. And, usually some teenager or husband was in trouble. Often I'd try and find a way to handle it discretely. But, sometimes a situation would present itself where I had no choice but to tell them exactly what I found and provide the options as to what they wanted me to do with it (such as when found items were the cause of the issue).
In one case a grandmother was passing a computer down to her pre-teen grandchild and I felt she should know what was on it before giving it to said child. I handled it delicately. But, I had to tell her. I couldn't let it go to a child without giving her the option of having me remove the information / files. And, legally I couldn't just delete stuff without first asking her permission (particularly since the files in question were not part of the requested repair but rather something I accidentally located while looking for something else).
What was always a bit interesting was when I'd be hired by a wife or mother to explore a husband's or kid's computers as thoroughly as possible to see if there was something going on. And, in the case of kids computers, the answer was almost always yes. In the case of husbands computers, it was often yes. And, it was always a little uncomfortable delivering my report. Though in many cases they were right behind me watching what I was finding. So, I only had to show them as I found it.
I found some interesting stuff.
Honestly, in most cases, just deleting a file is sufficient. Most people don't care beyond that. I don't. I've never gone over any of my used computers and looked for remaining data fragments. I just don't care.
But, if someone does care enough to try, understand that no amount of over-writing the data will guarantee security. All it means is that the more thorough you are, the harder they will have to try and the better the tools they use will have to be.
