Oftentimes, you buy something from a seller in your state, and they'll charge you the sales tax too.  Are you wondering if they actually turn it over to the state or not?  Well, they have to have a seller's license or permit in that state with a resale number on it, and if so, then they're required to make their monthly or quarterly sales tax reports and turn in the money.

But many times it seems that some sellers on ebay just like to collect the tax to raise their profits, and not turn it in. Currently, if they don't actually have a physical presence in the state, they're not required to collect that state sales tax on out-of-state sales.  Who knows if that will ever change, but as yet it's that way.

When I buy from a big seller in my state of Calif, I exspect them to collect sales tax.  If I buy from a little seller who doesn't seem to sell much and likely doesn't have a physical store, I'll ask them to give me the sales tax or seller's permit number.  Remember that every state requires that permit to be on display in a physical store, and for internet sales you have the right to have that number, and those with such a permit know they must give it out to buyers, even before a purchase, just as if it were on display in a physical location  If they hesitate, then that means they likely don't really have the permit and don't turn in the sales taxes, and if that happens, I just won't buy from them, and I tell them why.

Some stores collect sales tax for every state.  Supposedly they have a physical presence in each state. They still have to have a sales tax permit from each state to do that, and you can still ask for the number.  Once you get a number, you can confirm it with the state agency website that handles these things. For example, for Calif it is the state board of equalization.  You can report suspicious sellers to your state or any state if you think they aren't really turning in the sale tax.  After all, sales tax isn't supposed to be used as a method of increasing the sales price for profit.  I guarantee the particular state will check it out.