Yes if you sell on EBay that is self employment.  What 600 isn't taxed?  You might not get a 1099 for under 600 but all self employment is taxed.  Well just the Net Profit after expenses is taxed.  

I am trying to understand that for applying for disability, do I need to calculate so much tax to the payouts I get from what I sold on eBay and get the difference put that in? Or do I just leave it since I know I will be paying for the taxes of what was sold later on during tax season?