In an alternate universe all governments of earth believe that no-one should be born with their freedom and must pay for it in order to be classified as a full citizen and human. Before birth each child is injected with tiny nano machines which fuse with the cell during development and fundementally alters the baby's body. Once born the child is technically property of the state unless bought back.
The nano-machines serve many purposes, the main one indicating who are the "free" who are the "bought" and who are "for sale". These words are written on each persons forhead and can be seen through peoples eyes using the same nano-machines. This means that people use the nano-machines to reveal what others statuses are but have the ability to turn off this function so the words won't appear.
You are classified as "free" once your freedom has been bought and given to you making you the owner of yourself. You cannot be bought after this but you can sell youself back to the government for a sum of money. It is also possible to "rent" your body out to other people for a certain period of time and is common place in cetain occupations such as prostitution.
Once your freedom has been bought by someone you are legally no longer seen as a human and no longer have human rights in the eyes of your buyer. You are still seen as a person by others but your buyer has the power to do whatever they want with you and get away with it.
Everyone else is classified as "for sale" meaning that they belong to the government and pretty much have the same freedoms as "free" people. However, "for sale" people can be easily drafted without consent, can be imprisoned if the state orders it and must do anything the government says. These restrictions are why "free" people are seen as the highest people of society as normal rules often do not apply to them.
Age, sex and race also have no affect on the nano-machines meaning that a five year old child could buy a forty five year old man and that would be fully legal. That person would then have to do anything the child said. Even if that order is to commit a crime the slve must obey it and they'll be the ones to pay for it.
The amount a person is worth varies considerably from person to person and the mathermtics behind it is incredibly complex, so I won't get into it. Basically what the system does is it uses every source of media and networking to determine a persons worth and judge them by it based on where they are and who is buying them. If a person is incredibly famous then their price would be exceedingly high while a low life scumbag would be quite cheap.
A hot cheerleader who is the most popular and well liked individual in school would be expensive to buy from all the guys while the nerdy girl who is smart would be expensive to someone who values those qualities. In other words, the more valuable a person is to society the more that person is worth while the more popular they are in social circles grants them equal pricing. Think of it as a point system with the more points a person has the more expenisve they are. Like I said the mathermatics in complicated and hard to explain.
But more about what the nanomachines do. They show your status and price to others and vis versa but also affect your biology in other ways. They have the ability to completely take over you nervous, muscular and skeletal systems as well as all mental abilities and put them under the power of your buyer. Who ever owns you can make you do whatever they want but it goes futher than that.
These nano-machines can also alter your biology making you smaller, taller, stronger or weaker and can change anything about your biology that a person desires. The only draw back to this is that bio-altering costs a lot of money and is charged for every alteration.
For this reason your typical high schooler doesn't do this often unless they're super rich or something. Mostly, owners just use the nano-machines to completly control their slaves bodies as that is completly free.
And now on how this has changed society! This alternate reality has different social standards and significant changes to how the government operates. While being a slave will get many dirty looks from normal people it is good in that it prevents the government from harassing you unless they have legitimate warrent to do so.
Buying people is incredibly simple and often done in a split second. A person might be talking to their friends in a food court and a second later a stranger walks up, zaps then and then they are completely owned by that person. Although it is a shocking experience when it happens to you it is still seen as normal and quickly accepted.
The way people buy each otehr is that they walk up to the person that they want to buy and the nano-machines in each of your bodies "communicate". You see the price, you accept the price and the nano-machines instantly connect to your bank account and remove that amount of money.
There are many ways that a person might use their slaves. Teachers have been known to buy poor students just to teach them a lesson and use them as an example to the rest of the class. There have also been strange cases where a student has bought the teachers effectively ending class for that day.