This method in question is: Vending Machines.
You've definitely used one or more of these before. You put in $1-$2 in return for a snack or drink. It doesn't seem like much profit at first glance, but you are highly mistaken.
Most vending machines you see at the mall, store, etc, are owned by companies which operate on their own terms and machines, and in turn pay the property owners' either a small rent or % of sales cut.
~However, if you're able to install your own vending machine, you've struck a gold mine.
(The only downside is operating machine + contents by yourself)
I've personally seen just how much a vending machine can make in a single location. They basically pay for themselves multiple times over.
Here is the rough costs/profits of operating 1 vending machine:
Each item is $1, = $1.00 to $0.50 profit per sale.
Let's just say ever sale profits you $0.50.
Depending on popularity,
10 sales = $5 profit
25 sales = $12.5 profit
100 sales = $50 profit
Say per machine, per day, makes $40 in profits.
$40.00 * 7 days = $280.00
$40.00 * 31 days = $1,240.00
$40.00 * 365 days = $14,600.00
However, this is only one machine.
If you manage a vending machine empire, you may have multiple machines in a given area.
$14,600 * 10 = $146,000 /Year
$14,600 * 100 = $1,460,000 /Year
$14,600 * 1000 = $14,600,000 /Year
Even if you incorporate fees such as restocking, maintenance, etc, you would still be making bank off of your vending machines.
However,
If you're able to legally maintain a single vending machine in a populated area with your own labor, you'll make easy (almost) passive income.
This is a difficult yet highly-profitable field of competition that many of you have not considered, and many of you may actually have the opportunities to invest into a licensed vending machine.
Or, you can rent out another company's vending machine on your business' property.
Hope this helped someone!