Is it double taxation?

As S7horton says, the tax isn't on the item, it's on the transaction.

The tax being charged though is probably the transaction taking place between the seller and ebay...

IF your state has a sales tax you ARE REQUIRED TO PAY IT.
If not when purchasing off the net, then when you file your taxes... They ASK for the money. (Of course whether you cough it up is your lying, cheating soul at stake...
(8^Q...
Then one wonders what the seller is doing asking for a sales tax. ??Maybe he has been 'hurt' in the past by the tax collectors and wants to be certain he covers his a$$.
Or, he may just want to add a bonus to his bottom line.. as I have NEVER heard of private indeviduals, or non 'in your own state' requirement to collect sales taxes.

Avguygeorge is correct--the eBay seller is a dealer and must PAY salestax and is allowed to pass it to the buyer...at least that's the way it works in AZ.
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Your income is taxed 35-45% and then you spend it at a store and pay taxes on the sale. The business owner pays 35-45% income tax on that. He then spends the income at a store and it starts over again. For every dollar the Federal Gov't spends it gets back $8.00. That's why the government issues bonds/borrows money and then spends it.

"For every dollar the Federal Government spends, it gets back $8.00"
Then why do we have a deficit? The Federal Government should have enough money to not collect taxes for the next 1600 years.