Freelance services
Selling a skill by the hour or by the project: writing, editing, translation, bookkeeping, design, video editing, virtual assistance, customer support.
Why it works. This is where most beginners should look first, because you sell something you already have rather than building an asset and hoping. Demand exists every day of the year, the buyer is easy to identify, and the feedback loop is short enough to learn from.
How it usually fails. Competing on price against a global market, and accepting scope creep because you are afraid to lose the client. Both are pricing failures, and both are fixable in an afternoon once someone shows you the arithmetic.