How can the government charge for tax on something that was already taxed when new like a used vehicle?

How can the government charge for tax on something that was already taxed when new like a used vehicle?
The dealer or whoever is selling the used car (or any other used item) has to pay tax on everything he/she/company sells (if it is his living or if it is a company, if you sell one thing once in a you do not). He/she/company has to record how much they sell and pay tax on it, in some countries all the time in some yearly.