The home of the slave traders is found in volume 2 of the Kino Light Novels and episode 2 of the 2003 anime, under the name 「人を喰った話」(Hito o Kutta Hanashi"; A story of eating people)
The country of the slave traders has not outright been visited; nor is the name ever stated. At the end of autumn season, different teams of slave traders travel to the north to trade. Should every one get home alive, the country has a large celebration they call the Returning home alive festival. It is unclear if the country is nothing but slave trading or if they have other customs. They may or may not practice cannibalism like the ones that Kino met did, though that may have been out of desperation. They have a special song they play for travelers a little something for overcoming the hard ships of the road and their thankfulness for life, it's actually just to lull them into a false sense of security so they can kidnap them.
While little is known about the country, it has been said that it has never actually been visited nor is its real name actually said. Some of its people's traditions were stated to Kino during her dire time with the traders before they turned on her.
The country has a trail to the north that is used to trade their goods with the northern country. After the fall harvest, different teams set out to do trade—their main commodity appears to be trading slaves.
The country seems to do considerable trade in slavery as they send teams out every fall to sell and trade them with birds in the north. It also seems likely that they also capture them along the way. It's possible they use the pass as a good chance of running into others so they can trick and capture them. They may also eat the people they abduct, the ones Kino met did but this may have been desperation as they had been stuck out in the middle of winter with no food.
The returning home alive festival
Every year after the fall harvest people set out in teams to do slave trading and possible collecting. Given the danger of the trip and the practice if every one manages to get home a live they throw a special celebration called the returning home alive festival. There no set date for it to happen, as it only takes place on the day the last team returns home. It is a huge celebration with lots of food and woman(some may or may not be slaves). Festival days are sacred to them and they believe it is good luck to get married on a festival as it is said to bring happiness and prosperity.
Interaction with foreigners
They are far from hospitable, though again it is not seen what happens when someone goes to the country. but if one of their teams meets someone on the road they will not hesitate to kidnap them to be sold into slavery, even people who save their lives or bend over backwards to help them. They also have a special song they play when they meet travelers, a little something for over coming the hardships of the road and there thankfulness for life. It is actually a ruse to lull them in a false sense of security so that they can kidnap them.