There's instances where I've bought from nonprofit thrift stores and no they don't charge sales tax. I don't know why. Is it that because they don't have to pay sales tax as a nonprofit, neither do they collect it from the buyer? I guess so. But the simple answer is, if your state has a sales tax, you're required to pay it on any sale, or otherwise in any event where currency exchanges hands for goods or services.
Ebay is merely collecting sales tax on goods and services now for the respective states in which the purchase is made. Different states have different sales tax rates. They've all chomped at the bit to require online merchant sites to collect sales tax. Formally, all US states now collect sales tax from online sales in their states. They didn't all used to, but were growing jealous of the money they weren't getting. A lot of sales happen online anymore, fewer in traditional brick and mortar businesses. While it's convenient that ebay collects this sales tax from buyers instead of having sellers do it (and remit it to their states), you still ought to keep track of it, in case they make an error.
But I see where you're coming from. Someone buys a new car. Tax is paid. They resell the car. Tax is paid. Someone else sells that same car. Taxed, again. It seems unfair. But tax isn't a one time thing. It's a recurring thing that happens when currency is exchanged for goods and services (sales... tax).
Income tax on the other hand is a different thing. Depends on whether you made 'profit' on the item, or not. A sale of personal items you once already paid ie MSRP/full price for, when you go to resell it, you often don't make a profit. But if it's ie a collectable that has gained value, and you're making a profit by its sale (more that what you paid for it), it's taxable.
