I could not find anything saying it is our responsibility to total up our purchases and somehow pay at tax time.Bill, Google "sales and use tax" (with the quotes) plus the name of your state. That will lead you to an informational pdf on the subject issued by your state tax authority.
We buy stuff on Amazon etc...with no tax, but must keep track of all these purchases and somehow pay the tax to state we reside in.BTW, as you are probably aware most of the major online retailers make it very easy to print out a summary of purchases one has made from them during the course of a calendar year. Under My Account/Order History or some such link.
Best regards,
-- Al