Do I have to pay taxes on selling personal items?

I'm curious, do I need to pay taxes when I sell personal items? Is there a specific threshold or value that triggers taxation obligations? I'm particularly interested in understanding how the laws vary by country or state, and if there are any exemptions for certain types of items. Additionally, I'd like to know if the tax implications differ depending on whether the sale is conducted online or in-person. Can you provide some clarification on these matters?