Most creators spend all their energy finding new buyers and almost none serving the ones they already have. That's backwards. The person who bought from you once has already done the hardest thing — trusted you with their money — and they're far likelier to do it again.
The second sale is the easy one
Acquiring a new customer is expensive and slow. Selling something else to a happy existing one is neither. A second product — a follow-up, a deeper version, a companion — is often where a hobby starts turning into an income.
Build the obvious next step
The best second product answers the question your first one leaves people asking. Watch what buyers say they want next, and build that. You're not guessing at a market; you're serving one you already have.