Okay so if I sell $500 on Mercari and $100 on Poshmark…. Does that mean I have to fill out a 1099 because I reached total sales of $600 COMBINED? Or does my profit have to be $600 from each selling website?
It’s $600 for each site. But you don’t fill out a1099–the site does (1099-K)—which you report on your tax return. But the site will ask you to complete a W-9 for it so that it has the information required to complete the 1099-K for you
You don't fill out a 1099. The business paying you would give you the 1099. Or if you pay someone 600 or more (that works for you) you give them a 1099NEC.
Actually just enter your total income as Other self employment income or as Cash or General income. You don't need to get a 1099NEC or 1099Misc or 1099K. Even if you did you can enter all your income as Cash. Only the total goes to schedule C.
How to enter income from Self Employment