If you are engaged in an "ongoing trade or business" then you report the business on schedule C. "A trade or business is generally an activity carried on for a livelihood or in good faith to make a profit. The facts and circumstances of each case determine whether or not an activity is a trade or business. The regularity of activities and transactions and the production of income are important elements. You do not need to actually make a profit to be in a trade or business as long as you have a profit motive. You do need, however, to make ongoing efforts to further the interests of your business."
https://www.irs.gov/businesses/small-businesses-self-employed/business-activities
Basically, if you promote yourself as a business and are trying to make a profit, then you are a business and you report your income and expenses as such, even if you don't actually make a profit.
You are expected to keep reliable business records of your income and expenses, even if you don't get tax paperwork from your vendors and clients. Your business expenses can include your cost of goods (of course) but also other costs that are ordinary and necessary for your type of work. For a sales job, your expenses might be advertising, business use of a car or other vehicle, storage of inventory, and a home office if you use your home office for the administrative part of the job.
