My feeling is that "freelancing" is synonymous with "independent contracting" and refers to work performed for clients under a contract basis. For the work to be considered "freelance", the individual must be hired to perform work by third parties. Using the above example, Sally would not be considered a freelancer because she was not hired by a third-party to develop her app, but created it herself and is now trying to market it for sale. Under these conditions, I feel she would be referred to as an entrepreneur.
a person who works as a writer, designer, performer, or the like, selling work or services by the hour, day, job, etc., rather than working on a regular salary basis for one employer.
a person who acts independently without being affiliated with or authorized by an organization
a person who pursues a profession without a long-term commitment to any one employer
working independently usually for various organizations rather than as an employee of a particular one:
A freelancer or freelance worker is a term commonly used for a person who is self-employed and is not necessarily committed to a particular employer long-term. Freelance workers are sometimes represented by a company or a temporary agency that resells freelance labor to clients; others work independently or use professional associations or websites to get work. "Independent contractor" would be the term used in a higher register of English that designates the tax and employment class of this type of worker.
An independent contractor is a natural person, business, or corporation that provides goods or services to another entity under terms specified in a contract or within a verbal agreement. Unlike an employee, an independent contractor does not work regularly for an employer but works as and when required, during which time he or she may be subject to law of agency. Independent contractors are usually paid on a freelance basis.
a person or business which performs services for another person or entity under a contract between them, with the terms spelled out such as duties, pay, the amount and type of work and other matters.
A person or business who performs services for another person under an express or implied agreement and who is not subject to the other's control, or right to control, the manner and means of performing the services; not as an employee.
The Freelancing stack exchange currently does not include a detailed description of freelancing. However, the help page "What topics I ask about here?" indicates:
If your question covers...
•Agency work while freelancing
•Selling services to different employers without a long-term contract with any of them
•Client responsibilities to freelancers
•Cross-border requirements and laws for freelancing
...then you’re in the right place to ask your question!
None of these suggests that questions about marketing and selling ones' own products would be on-topic.