Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.
In response to your question, I say:
The state of slaves present today in the world is not known neither is it known if these were slaves before. What we hear today is that many of them became slaves as a result of deceit or enslaving free people. Whatever the case might be, it is permissible to buy slaves from the disbelievers because the principle on slaves is that it is permissible to buy them from disbelievers and accepting them as gifts. The Messenger of Allah (peace be on him) accepted Maria as a slave when she was given to him as a present by the king of Egypt. This story is famous in history and translation books. Moreover the principle on transactions is that they are permissible. To this end, Allah the Exalted said: “whereas Allah has permitted trading and forbidden riba (usury).” [al-Baqarah: 275]. None the less, ‘Umar (may Allah be pleased with him) disliked buying of slaves from disbelievers. Al-Baihaqi transmitted in his Sunan (9/140) on the authority of Umar (may Allah be pleased with him) with an authentic chain of narrators that he said, "Do not buy the slaves of the Thimmis because they are a people upon whom the jizyah is due, and some of them pay on behalf of others." The scholars differed on the reason for the detestability, and the strongest reason is what was mentioned by ibn al-Qayyim (may Allah be pleased with him) in Ahkaam Ahli ath-Thimmah 1/133, "It is as if 'Umar (may Allah be pleased with him) did not establish for the slaves of the Thimmis the rulings which are established for the slaves of the Muslims. He knew that some of them would sell others, and that does not establish slavery in reality. Thus, he prohibited the Muslims from buying them as a precautionary measure. Then ibn al-Qayyim said, "Ali ibn Abi Talib (may Allah be pleased with him) agreed with 'Umar, and he detested that Muslims should buy the slaves of the Thimmis. Sa'eed said, "Qatadah detested the buying of the slaves of the Thimmis by the Muslims. He allowed the buying of the slaves who were not from the countries of the Thimmis like the blacks, the Abyssinians, or the Khorasans who did not sell each other.” In conclusion, buying of slaves was disliked due to uncertainty of slavery. Whoever wants to take this precaution it will be for his good and there is no harm in buying for the one who does not want to take the precaution. However, it is not permissible to buy if you are certain or in great doubt that what is being sold are not slaves but rather free people being enslaved.
And Allah knows best.