To calculate that value, you take the amount you pay for the good, multiply it by 0.4 and then add the amount you pay for it. If manufacture cost = c and store value=v, then 0.4c+c=v
That is the amount of money you pay for a brand new item. You may see some price-fluctuation between, say Canon and Nikon versus Amazon.com or B&H Photo and Video. What you can presume from this is that, since they are all businesses, they are all still making a profit (duh - how could it not be that way?), and that's good for us because it means we still have somewhere to go to buy things. We can also presume that either the lower price is less than the 40% to stay competitive, or Canon and Nikon are making more than 40% because they are the name-brand store.
If you use the 40% rule in reverse, you can find how much it cost to make that brand new good at cost. IE subtract 40% of the value from the store value instead of adding.
In order to purchase a used camera, the price being offered better be less than the cost it took to manufacture it, or else you're getting ripped off, in my opinion. Presumably a second-hand camera has the risk of coming with some defects, and presumably you don't know what those are because you don't know how it was used, when it was abused, etc.
There's another reason: periodically, when businesses have a surplus of items that are not selling (like I have presume will happen with the Canon 5D Mark ii cameras) you will see them decide to make less than 40% profit off that item in order to clear it out and make room. Maybe it will be 10% profit - I've seen that number out there. That is when you will see a brand new camera marked at the cost you see some people who don't use this math equation pricing their second-hand or refurbished cameras. Had I just bought a used camera for that price and saw it listed at that value new, I'd feel like an idiot!
I am patiently waiting for this low discount to happen so I can get a new camera truly CHEAP, but if I see someone asking a FAIR price for their second-hand camera, I'd consider taking them up on their offer instead. I would never pay more than this low-ball 10% profit for new value for something another photographer is selling used, and I've seen lots of photographers asking high values. I couldn't decide if it was ruder for me to comment on that high price and warn potential buyers, or if it was ruder for me to not comment and not warn people. So instead I wrote this article.
If you don't want to be ripped off, you should take the time to figure out the math and know what the used camera is worth. It's kind of like buying a used car.