"Mass" is the amount of some substance you have...the number of protons, neutrons and electrons. But many material objects are sold by weight or by volume - neither of which are a constant representation of the amount of "stuff" you're buying. Whenever someone makes a mistake like that, there ought to be a way to make money from their error!
Here is an interesting idea. Because of centrifugal force, and the fact that the Earth isn't perfectly spherical - the force of gravity is about 0.7% greater at the poles than it is at the equator.
The price of gold is around $40 per gram. If you buy a kilogram of gold at the Nairobi gold exchange (close to the equator) - it’ll cost you around $40,000 and you could fly it to some northern country like Iceland where it would weigh about 0.7% more - which is 7 grams…so on paper, you just made $280 profit!
The most obvious problem is that the airfare between the Equator and somewhere sufficiently far north (or south) is a lot more than $280. But that's OK - even if the air fare is $1000 - you can just buy ten kilograms of the stuff - make $2,800 and you have a healthy profit - right? 10kg is within your baggage allowance.
The second major problem is that you can’t sell gold for the same price you can buy it. The dealers have to make a profit - so they’ll either charge a commission - or they’ll simply post different prices for buying and selling. I don’t know if that would wipe out all of your profits…but I do know what would!
The real problem is that Gold buyers don’t measure the WEIGHT of the gold - they measure its MASS. The mass of the gold doesn’t change depending on where you measure it...it would be the same on the Moon as it would be on Earth.
To do this, they might use a balance with a scale pan and some calibrated weights. What this means is that when you put 1kg of gold on one scale pan and a 1kg weight on the other - the balance beam balances - no matter whether you’re at the pole or the equator. Your gold changed weight - but so did the 1kg weight on the other pan.
A balance beam doesn’t ascertain the “weight” - it tells you the “mass” - which is independent of the force of gravity.
If they do use an electronic machine - they’ll first calibrate it to their local gravitational force by putting a known 1kg weight on the device and adjusting it until the readout is exactly 1kg.
So in either case - you won’t win a penny.
You really need something that's genuinely sold by weight...iron maybe? But then the cost of shipping it kills you. So perhaps this isn't a great approach?
In Texas (but not all US states), gasoline is sold by the gallon. This is pretty stupid because (like gold), it should be sold by mass. What you care about is how much energy you're buying - and that is a matter of the number of molecules present - and that's measured as mass - not weight or volume.
So - buy gasoline in the depth of the Texas winter and sell it again at the height of summer.
Gasoline expands by about 0.1% for every degree centigrade of temperature rise. There is about a 40 degree rise between the coldest winter day and the hottest summer day in a typical year in Texas - so if you buy a gallon when it’s freezing outside, then by mid-summer, you’ll have about 4% more gasoline (by volume) than you started with!
4% over 6 months is a far better return on investment than our failed gold scheme…and probably better than the return on investment than you’d get over 6 months on the stock market!
Of course gasoline "gets old" but what really happens is that the lighter, more volatile parts of the gasoline mixture evaporate away, leaving the heavier (and less energetic) parts behind. But if you kept the gasoline in an airtight, pressurized container, that wouldn't happen.
So - there you go. Go make your fortune!