I sell personal items online. At what point does my income from that become taxable?

Your income from selling online becomes taxable when your profit is 50 cents or more. It makes no difference whether you get a 1099 or not. Getting a 1099 is not what makes the income taxable.

However, if you sell personal items at a loss (i.e. you sell something for less than you paid for it), you have no taxable profit (and you cannot claim the loss). But if you get a 1099 you will have to report the income on your tax return. It can be reported as "personal items sold at a loss" so that you don't have to pay tax on it, but you still have report what's shown on the 1099.

These are federal rules. They are the same in all states.