Net income divided by net sales equals operating margins.
In other words, operating margins represent how much a business needs to sell things to make money.
Here's a few businesses with interesting margins:
Credit Acceptance Corp: 69%
Canadian Pacific: 52%
And here's a short list of great businesses with low margins (7% margins or less).
Alimentation Couche-Tard: 6%
For that list, I've retained only 4 stock known by everybody. There were many obscure names on my screening, which is normal: if a business has poor margins, it's probably a poor business. So, you don't hear about it.
Obviously, If there's some money to make, entrepreneurs will be interested to go start a business in that specific field. So, there should be some competition eventually. But it's not a good reason to avoid to take a look at high margins businesses first.
Few people look at margins first. It's not the first thing to check when you analyze a stock, but it's interesting to see the challenges that faces the business (margins are one of these challenges). And it's impossible not to be amazed by Couche-Tard that achieved such a spectacular performance with such low margins.