You don't need a business license to charge 
state sales tax, what you do need is a state sales 
tax ID number in order to collect tax.    
Normally sales tax is charged only for in-state 
sales, unless the seller has a dual-residency 
and has a tax ID for another state.  
As far as over-charging for tax, yes that's illegal, 
however different cities and zones have 
different tax rates.   Also, many sellers are 
unaware that they can only charge the tax rate 
of the location within the state that they are 
mailing the item to.   Example, you live in Phoenix 
and the seller is in Tempe, they must chager you 
the sales tax for Phoenix.  It is a bit extra paper 
work when the seller files their taxes, that's 
probably why many just "pretend" they don't 
know about it.  
Also, those that charge tax at conventions, must 
charge the local tax in the city that the 
convention is in.  I noticed a lot of sellers 
charging tax for the city they came from, which 
is illegal, but like I said above, many either 
genuinly don't know this, or pretend they don't, 
in order to avoid the extra paper work.

Evil Jew