In many countries the local municipality (or county, for rural areas) government has relatively significant powers: they can pass local ordinances, raise taxes, regulate certain activities, etc. Assuming one had enough money and willing buyers, would it be possible to purchase land of an entire municipality and take over the local government as the sole legal resident?
To avoid making this too broad, I'll limit the scope to the United States.
It's not that simple, and it is.
Residency: Merely selling your property doesn't make you no longer a registered voter in that area. After you move to a new area, and establish residency, you can register to vote in a new area. That's when you would lose your ability to vote based on your old address (I think). We'll assume you can handle that as a detail.
Special districts: There are a lot of other governing structures that cover county lines, cross between cities, etc. For instance, a school system may cover multiple counties, or a transit authority. All of these can collect taxes, and otherwise impose rules. There are probably more of these then you think - I think they outnumber counties like 10 to 1 in the US.
Minimum services: I think you're required to maintain certain services as a county/city, even if they aren't needed. For instance, a municipality may have to provide fire, police, trash, etc. services or be dissolved. This may require more residents, although I think they can all be people who commute.
Longevity: Municipalities and counties, at least in the US, exist because the state allows them to. They can be dissolved by the state legislature. So, if you pass insane laws, prepare to either be dissolved, or overruled.
However, if you have that much money, you can probably work with the state to establish what you want. Look at Disney World. It has a special district with lots of unique rights. It's immune from state inspections, has a city ruled by, IIRC, non-elected disney employees.
If you purchase all of the land in a community - assuming all of the land is for sale - and don't rent the properties out, then you are the only voter. You can vote yourself in as mayor, and pass whatever laws you have. Of course, that also means you're the only taxpayer, too, so your budget will be quite limited.
If you rent the properties out, then you're not the only resident or the only voter. If you rent out only to people who will vote how you want them to, you'd be violating several federal laws regarding equal opportunity housing, and voter tampering or fraud.
Even as a sole resident, you will still be subject to state and federal laws, though. Most states hold that state law overrides municipal law, and federal laws definitely override state and municipal laws when the laws are conflicting.
So if you're thinking about buying a town and becoming a financial haven for tax evaders and money launderers... sorry, state and fed laws still apply, even if you are the only voter.
