There is a big argument going on at the moment about the buying and selling of rhino horns.
Currently, buying and selling rhino horns between different countries around the world is illegal.
However, a legal sale is taking place in South Africa, which will last for three days. Because it is is taking place within this one country, this kind of sale can be legal.
But there is a big argument about whether or not it should be allowed to happen at all.
Read on to find out more.
Rhino horns are extremely valuable - even more expensive than gold.
The main place where people want to buy rhino horn is Asia. Some people there believe that it can be used in medicines and to cure diseases, but there is no evidence that says this is true.
Because it is so valuable, criminals kill rhinos illegally in order to take their horns and sell them for lots of money.
This is called poaching - and it is a huge threat to the number of rhinos left in the world.
Currently, more than a thousand South African rhinos are killed every single year because of poaching.
Environmental groups are worried that if something isn't done to stop the rhino poaching crisis, the species could die out forever.
Buying and selling rhino horns between countries is against the law.
But in April this year, the most important court in South Africa ruled that sales of rhino horn could take place, as long as the seller had an official permit and that the sale happened within the country's borders.
Now, a man called John Hume, who owns the largest rhino breeding farm in the world, has won a series of court cases to allow him to sell some of his own rhino horns in South Africa.
He has tonnes of horns that he has removed from animals on his farm. The way he removes them is painless for the rhinos, as the animal is put to sleep for the procedure by a vet, and the horns do grow back.
Now, he wants to sell the horns that he has removed.
John believes that buying and selling rhino horns like this - in a legal and controlled way - is good for the animals because it can raise a lot of money, which can be used to help to protect rhinos and save them from extinction.
He also believes that having more sales like this - and making horn available in this way - will bring down the price. By making it less valuable, he thinks this will discourage poaching.
"Poaching is out of control in this country," he says. "I firmly believe that this is the way to save rhinos from extinction - to breed them better, to protect them better."
"One of the ways to protect them better is not to make the horn completely unavailable to everybody."
Many environmental groups and conservationists disagree with John's way of thinking.
They believe that sales like this will encourage more people to buy more rhino horns, which means the animals will continue to be poached.
They think more domestic sales will encourage more illegal international buying and selling of horns between different countries - and that this also will encourage more poaching.
Dr Kelly Marnewick from the Endangered Wildlife Trust says: "There's been absolutely no evidence shown that trade is going to [decrease] rhino poaching whatsoever."
She also thinks that if there is more legal rhino horn available to buy and sell, it will be easier to hide illegal rhino horn among it - so illegal buying and selling will still carry on in secret.
Those against the sale say there aren't many people in South Africa who actually want to buy rhino horn, as the main place where people want it is Asia.
So they believe that, in the end, the horns being sold in this sale are due to end up in Asia - which isn't allowed.
Many people on both sides of the argument will be watching this sale - and what happens after it - very closely.
If rhino numbers go up, then people like John will be happy and more sales like this might go ahead in the future.
But if rhino numbers keep going down, then conservationists will say that sales like this don't work, the rules need to be stricter and more needs to be done to protect this species.