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.