Slavery existed in China, primarily before the 1st Imperial Dynasty of Qin, which unified China as an empire.

Prior to the Qin Unification 232 B.C., various feudal Chinese Kingdoms practiced slavery to some extent, where slaves are considered to be property, (1) can be bought /sold, (2) can be killed by owner, sometimes sacrificed ritualistically, (3) have less legal protection than citizens.

However, during the 550 years of civil war before the Qin Unification, (Spring Autumn period and Warring Nations period), Slavery fell out of fashion in China, for many reasons:

(1) logistic difficulties: Slavery requires extensive tracking of identities of slaves. With constant war in China, it became simply impossible to keep track of all the slaves for very long. Rival Kingdoms would not necessarily recognize each others' slaves. Slaves who ran away could easily gain freedom by pretending that they were free all along.

(2) Easy ways for slaves to gain freedom. Even in that time, Chinese began some basic forms of meritocracy systems, where even slaves could gain freedom or title by achievement. As war grew, many Chinese Kingdoms enlisted slaves as soldiers with promises of freedom.

(3) Rise of classic Chinese philosophies. Many classic Chinese philosophies preached against slavery. Legalism taught that people should be equal before the law. Legal reforms introduced by Qin China effectively freed all slaves, simply because the Qin Emperor didn't want to share the power over life and death with Feudal lords. Confucius also emphasized that Education and Virtue are more important than a person's birth status.

(4) Population growth. China experienced significant population because of its advanced in Agriculture. It made slavery non-economical as a way to increase labor.

(5) Communal approach to organized production. Chinese communities organized efficiently around communities of clans of related families. Farming, industries, trade, all took on approaches/processes around this type of familial organizations. Introduction of slaves would actually disturb the normal efficiency of the process. Slaves would thus be regarded as more of a burden than a "help".

(6) Confucius abhorrence toward "barbarians". Post Unification Qin China feared the "Barbarians", and tried to avoid them at all costs. Chinese Emperors thereafter, rarely bothered to send military expeditions outside to tame the Barbarians, and thus, rarely ever had any opportunity to take slaves from outside (unlike the Romans).

Technically, slavery didn't official "end" in China, until the 1900's.

Occasionally, some slaves were still traded in China, but the laws were very murky, somewhere in between legality and illegality regarding slavery.

However, slave trade never reemerged in significant size in post Qin China, because simply there was little or no market demand for slaves.

slavery was relatively low in these countries because they (especially China) had a large population which meant labor was abundant. When labor is abundant, the wage rate (aka cost of employment) tends to be lower than the cost of enslavement. therefore in principle, slavery was not really considered because it was not profitable. in fact, i would argue this is the most important single act to destroy slavery over night.

You don't mention Korea, but its relationship to slavery is interesting.There were formerly a a class of servants called nobi, who constituted at the peak perhaps 30-40% of the population in the 14th to 16th centuries. They could be bought, sold, given away and inherited. They could normally be physically abused without penalty even after laws were brought in protecting them. Their situation certainly meets the modern western definition of slavery, but Koreans, including historians, have tended to deny that that is an accurate description.

The ending of the nobi system seems to have been brought about by changing economics rather than any moral argument. Government nobi were freed in 1801 and by the time the institution was legally banned in 1894 there were few privately-held nobi left, although individuals were not freed by that law and some remained in service for many years more. Modern Korean society has ideas of the importance of equality that have been influenced by the experiences of the nobi and the large and also disadvantaged peasant class (who are sometimes also described as slaves).

A paper entitled Korean Slavery is here and a blogpost from a European historian on the question of whether nobi were slaves is here.