A hard core online gamer styling himself as “Demon Lord Diablo” gets summoned to an alternate world by an elf and a cat girl who accidentally get enslaved by Diablo while trying to enslave him.

Four men are summoned to an alternate world to be heroes. Three get a weapon (sword bow spear), but the fourth gets a shield and becomes the hated “Shield Hero.” So he strikes out on his own (and quickly picks up his first tanuki slave girl to fight for him).

A game programmer is transported to a fantasy world. He uses his overpowered skills to save people, rescue slaves (a mix of human and non-human), and becomes the center of attention of an ever increasing group of women and girls.

A class is summoned to another world where they become heroes with powerful hero skills. One student gets a basic matter manipulation skill. This student is betrayed and falls deep into a dungeon. There, he gains incredible skills, rescues a vampire, partners up with her, and together they search for a way back home to his world.
This isn't so much a master-slave relationship manga like the rest here, but there is slavery in this world, and the characters are prone towards the same tropey behaviors, too.

Magic portals leading to a demon world appear randomly across Japan, but only women develop the magic power to stop the demons. One man accidentally crosses a portal and is rescued by a member of an elite troops unit who enslaves him to draw out his life force to save their lives.

Takashi Izu is transferred to a world resembling the online game “Argate Online.” He becomes an adventurer. Soon, he rescues an elf slave who follows him, moves in with him and becomes his partner. And he visits a slave market.

A man is transferred to a game-like fantasy world after trading his memories for skill points. He joins the adventurer’s guild and soon stumbles across a half-elf being sold in a cage. He spends two seconds pondering the evils of the world, then gets on with his new life. Eventually he decides he needs to buy that half-elf female slave to strengthen his adventuring party.

A school bus of students is summoned into another world. While most choose to become heroes and train to defeat the demon lord, one student refuses and leaves to make his own way in the other world. He is immediately contacted by demons, and he agrees to buy and protect a demon girl slave in exchange for knowledge.

A group of students using an educational virtual reality simulator are transferred to a game-like world. One of the students becomes the demon lord, and a fellow student (and his boss at work) becomes a slave in the demon lord’s castle.

A university student is stabbed and is reincarnated in another world. After being fired from his healer’s job, he buys an elf slave to help him earn money in the labyrinth.

A class dies on a field trip, and one student is reborn in another world with the rare magic ability of slavemancy.

I don’t know how this isn’t an adult comic, but the manga magazine that publishes it has lots of erotic content, so maybe it is.

An elf slave and a lycanthrope slave fight over the hero who owns them.

A student is suddenly transported to a fantasy world and discovers he has the magic classification of “inspector.” He joins the adventurer’s guild who tells him to buy “some slaves” to help him earn money. So he does. This is one of the rare stories where a slave is male (but still non-human).

A man is offered an evaluation job by an employment agency. The job is to evaluate a fantasy world before it’s destroyed. He becomes an adventurer, visits a slave store where there are human slaves, but picks the cat girl.

A bus full of students are abducted to an evil fantasy world and forced to become “sage apprentices.” Lots of violence and casual slavery.

A blatantly over-the-top fan service manga where five friends are summoned to another world to defeat the demon lord. Four are heroes, but the fifth is an accidental “normal” person. The fifth gains super powered skills (cheat skills) and is transported to another place where he rescues a slave trader, buys a cat demi-human woman, and he decides to be an adventurer. <

This is all about a guy who wants to gather a slave harem after being transported to a fantasy world.

Summoned to another world, a man gains the “skill taker” power and gathers a slave harem. Lots of fantasized sex. And he becomes and adventurer, too.

A salaryman and his boss are killed in a buss accident and are transported to another world. They team up to fight in the dungeons, but only after he rents a “sub-human monster” slave woman.

A man is hit by a truck while on his way to his 101st job interview. He is reborn in a fantasy world with the ability to earn experience at 400x the normal rate, but without a job class. He quickly meets a slave trader (But it’s all for the slave’s well-being. He’s not a bad guy, honest) and buys a wolf demi-human woman to be his partner for adventuring (and to save her from a noble who wants to buy her).

Not only does Kazura win the lottery, but his parents house has a magical door to a fantasy world. Kazura travels back and forth between worlds helping out the people there. And a human(!) slave woman falls in love with him. Like they do. Like all the women there do.

A man is reincarnated as a magic sword. A young cat slave girl finds him. He kills her very bad owner. They team up and she names him “Master.”

A son is killed by his father, and the son’s soul is scooped up by an evil woman in a fantasy world. She turns him into a golem to be her slave. Yet somehow she’s the one who keeps getting put in bondage. The son eventually gets a slave of his own, too. She’s some kind of automaton, of course.

Maruta is a shut-in who is summoned by an evil goddess who wants to use him in her power games. However, she finds him lacking and dumps him in a fantasy world where a good goddess gives him a magic eye. His first visit to a town results in rescuing three naked slave women in a well (and they are human!). They become his followers. He joins the adventurer’s guild. He turns out over powered. His preferred weapon and special skill is as a giant log wielder (no metaphors there at all, right?)

A man is stabbed, dies, and is reborn in another world with a magic eye and a rifle. After saving a noble girl, he asks the girl’s father for information about the town they live in. The noble’s servant immediately suggests buying a slave. So, naturally, he buys a traumatized and disfigured rabbit girl whom he heals and puts to work fighting with him.