How do I enter income that has no forms from reselling items on platforms such as mercari, marketplace and poshmark? I do not exceed 20,000, so I don’t recieve any 1099.

This is my first year as a reseller. I was told to report even if I don’t exceed 20,000 but I have no idea how to get total amount of what I made from platforms because I have no received any forms.  I also am not sure how to deduct what I initially paid for these items. Please help! Is this worth sitting down with a professional?