In Canada and presumably the USA, sales taxes are actually "Consumption" [a.k.a. "use"] taxes. So, the consumer is the one who pays the tax on items consumed or used. For example, the contractors who build a house, pay the tax on all of the personal tangible property (i.e. nails, windows and wood) that they purchased that became 'real property' once incorporated into the build.
As a stamp collector, you have to pay the sales tax on taxable items you consumed in the state (or province) regardless of where it came from. You would have paid tax on the same item if you had purchased it from a local stamp dealer.
The state has required the supplier, i.e
ebay, to register as a collector of the state's sales tax, so applicable sales must be charged on taxable goods sold to a consumer in the state. Failure to do so, would result in
ebay's legal liability to pay the state the tax it didn't change and/or collect from the consumer.