Sales tax may not apply to online sales in the US (that is changing, and depends on the state involved and whether the company has any presence in the state). Income taxes are never incurred because of sales, only because of income. To oversimplify: no net income -- no income tax.

That's not quite true. I believe in every state you are supposed to declare goods you buy online and didn't pay sales tax for in your end of year taxes. I don't think anyone really does this and it hasn't been enforced.

The current fight isn't over whether online sales should pay sales tax, but over who should be in charge of collecting that tax: the seller or the buyer.

It's more complicated than that. Absent SSTP, if I live in a town with a special sales tax, and I buy furniture from the next town over and have it shipped to my house, I get to pay a lower tax rate. With SSTP, they furniture store has to track my address and charge me the higher rate. I don't think use tax has ever been a factor in intrastate sales tax rate variations so this does change what sort of taxes are required.

If I sell something on eBay do I pay income tax on it? No, though I suppose I would if I were selling a whole lot of stuff. Similarly, I don't pay sales tax on things I buy on eBay even though I'm supposed to.

1) eBay reports aggregate sales >$600 to the I.R.S. 2) Most items sold on eBay are sold for less than they were purchased (i.e., anything used), so the likelihood of recognizing gain on any sale is low. No gain recognition = no taxes incurred. 3) Sale of items above (your) purchase price do incur taxable income. If you do not report such income, your tax year never closes. That means there is no deadline as to when the IRS can go after you for penalties and back interest. 4) Sales tax is collected by the seller on behalf of the buyer. You mean "use taxes", which are paid by the buyer himself.