I guess sales taxes would be firearms related and concerns many of us enthusiasts due to the internet. When you buy a gun other than locally you can incur three costs that can really drive the price up:
1. FFL transfer fee
2. Shipping cost
3. Sales taxes
Taxes are always and interesting subject. Since we are talking sales taxes, keep in mind only states and municipalities charge sales tax. There is no federal sales tax (At least not yet) Plus some states have no sales tax at all.
In most cases. Licenced dealers must collect sales taxes if the sale is in their state. They can also collect sales taxes form a sale in another state if they have a franchise in that state; if that states law days they must collect.
Private transactions between individuals do not require collection of sales tax