Well, more specifically, DougFNJ (and more nit-pickingly

), the vendor is only required by law to
collect the sales taxes for the state he
operates in. The state is what
charges the tax.
In your example, NJ requires Mom&Pop.com to collect the sales taxes it charges you (as one of its residents), but TX has no authority to
require that Mom&Pop.com collect the sales taxes it charges me (as one of
its residents).
In that scenario, TX
does have authority to directly charge the sales taxes on me, but lacks the seller as an agent for collecting them from me. In fact, TX does still consider itself owed those taxes, and if they ever figure out a way to document and track them, I expect they'll send me a bill and probably a nasty note.