According to United States law, an ordinary person is allowed to own a decommissioned battle tank, and in fact, you can easily find them on the high-end market. The main catch is that the tank is inoperable – its engine and treads can work, but its cannons can’t, and you’re not allowed to buy or own its weapons. This is the reason why many removable tanks are sold in non-functional countries. If you want to drive around, you have to fix it yourself, but if you do, you can drive all you want. A few states in the US allow tanks to be driven on public roads with proper sidewalks, although in most cases, you should keep your tank in a private area.
If you want to know about the technology, a normal person can have a well-functioning tank, including missiles. In order to do this, you need to have a license issued by the government, and the government does not give you one of these because you asked nicely. Even if you have military training, you have to go through the official checkpoints, and even if you have a license, you are allowed to use the cannon in real situations. Overall, the difference is not justified.
A tank is an option if you are very wealthy and want a lawn ornament – an expensive, unpleasant option.