What is the cheapest way to get the first hundred customers?

People who already know you, and communities that already exist. The first hundred are almost never bought profitably, and trying to buy them is how new stores spend their entire budget learning nothing.

The reasoning

What works: selling to your own network without embarrassment, being genuinely useful somewhere your buyers already gather, and one cooperation with somebody who has the audience you want. All three also produce feedback that advertising does not.

What most people get wrong

Selling online is not passive income. It is a business with stock, customers and a support inbox, and anybody describing it as passive is selling a course.

The number that decides whether a store works is not revenue, it is what is left after the cost of the goods and the cost of getting the customer. Plenty of stores with impressive revenue make nothing.

Repeat customers are the whole game. Winning a customer costs money; selling to them again costs almost nothing.

How an app changes it

This is where an app earns its place — it exists to make the second and third purchase easier, which is where the profit is.