In some states, there is no sales tax on used vehicles (Arizona e.g.) In such a state, iif the sales tax is paid, you don't have to pay it agian. However, In other states, California for example, sales tax is assessed every time ownership changes (sellers/buyers don't always realize this). At 7-8%, in many states, it makes a pretty big difference, Also, it some states the license fee is referred to as a (property) tax, so could also mean this.
ps, for expensive cars, it also may mean the luxury tax, which is being phased out.
In Mass you have to transfer title within, I think 30 days of sale. This is to assure the state gets their 5% sales tax. You don't have to register the vehicle though. In some cases the buyer pays the seller the tax, like a licensed dealer, who then forwards the fee to the state. This can happen when buying out of state. The state would then forego the tax. ie; Taxes paid! (I think)
As mentioned above in a couple of posts... The "Harley Tax" is a phrase that means you must take a crowbar to the pocket book to make them run as they should have from the factory. Intake and exhaust upgrades followed by rejetting is the norm.