If you sell goods on Amazon.com, or eBay.com, and do so with the intention of making a profit (in other words they are not a casual reseller of used personal goods or other items that you may already own, then this is considered a self-employment activity by the IRS.
And unless you are incorporated as business, or are in a partnership arrangement with one or more other persons, you will need to file a Form 1040 Schedule C (and Schedule SE) to report this self-employment sales activities, as well as to deduct valid cost-of-doing-business expenses . . . such as cost of goods sold inventory, postage, Amazon commissions and fees, etc.
In TurboTax, you would need to use either the (online) Self-Employment edition of
the program, or you would want to use the (desktop) Home & Business
edition, to complete these tax forms.
While it may indeed be true that Amazon or eBay may or may not send you an annual tax reporting form of any kind (and it is unknown outside of Amazon and eBay what information or data they share with the IRS, or to the many various state departments of revenue), there is still a legal obligation on the part of the taxpayer in question to report his or her self-employment activity. This is true no matter the actual dollar amount of money involved.
Here are some TurboTax webpages that can help with more information:
https://ttlc.intuit.com/questions/1901340-what-is-schedule-c
So, basically, what you will want to do is create a Schedule C (and Schedule SE) in the tax software and report your self-employment sales. Even if you experienced a net loss, rather than a profit, the only way to tax that loss as a tax deduction on your return is to fill out a Schedule C (from which the information then "flows" back onto your main Form 1040).
If you have further questions, encounter any difficulties, or just want to speak with a live tax expert who can walk you through any Schedule C (business related) TurboTax entries, including the ability to screen-share with your computer, please feel free to contact us. We would be happy to help you. As a paying TurboTax customer, the call is free to you. Here is a link where you can create a support ticket and reach us:
https://support.turbotax.intuit.com/contact/