The major problem with this question is that for the most part the colonization of Africa and the slave trade happened during two different periods. The Atlantic slave trade went from the 16th century to about the 1870´-1880s. The colonization of Africa, with few exceptions, went from the 1880s until the mid to late 20th century, when Africa finally became independent of European colonial powers.

The Atlantic slave trade was based on key military outposts on parts of the African coast which would serve as commercial centers for exchanges between Europeans and local Africans. If a war was won, people would be enslaved and then sold to Europeans who would pay in clothing, alcohol and, significantly, European weapons. This would further strengthen local African groups loyal to the Europeans who would expand further, enslave more people sell them, make even larger profits. Other good were also exchanged, such as European goods for African gold, but your question was specifically about slaves so I´ll side step other aspects of this exchange.

An important exception to this would the South Africa, which was colonized by the Dutch in the 17th century. While other colonial outposts may have existed, they were the exception to the rule, and as in the case of Dutch South Africa the territory Europeans controlled was a lot smaller than the actual borders of the same country today.

Beginning in the first decades of the 19th century the British began to pressure other countries to abolish the slave trade. The British and the US abolished the importation of slaves in the first decade of the 19th century, with many other countries following this policy in the decades to come, largely thanks to British pressure. In countries where slavery was still in use, illegal slave trading was the norm though.

Then slavery began to be abolished. Haiti had abolished slavery decades earlier, but they´re the exception. The British abolished slavery in their colonies in the 1830s. Other countries did the same, such as the US after the Civil War in the 1860s. Finally, Cuba and Brazil abolished slavery in the 1880s.

By the time of the Berlin Conference in the 1880s the Atlantic slave trade was all but dead.

When Europeans colonized Africa, of course, this breathed new life into de facto slavery in territories they controlled, but your question seems geared to the Atlantic slave trade in particular.

In regards to your question as to whether some ethnicities were thought to be better, more hard working, or worse: Yes. This was definitely something slave traders and plantation owners talked about. From the primary sources I´ve seen in cited in the case of Cuba, there was a debate about which "tribes" tended to give the most trustworthy, obedient slaves. Of course the names and categories used by Europeans were very imprecise and referred to entire regions, despite sometimes obvious ethnic differences which foreigners never seemed to pick up on. I´ve seen some historians refer to territories where Islam had flourished as being notorious for rebellious slaves, but I the authors I´ve seen cite this seem to be making inferences of their own rather than citing contemporary sources who said this more explicitly.

To start understanding this period a great book would be Eric Williams´ Capitalism and Slavery. Regarding the attitudes of slavers to distinct ethnic groups I distinctly recall Manuel Moreno Fraginals talking about it in The Sugarmill, but only in the post-1973 edition. A lot of the English translations are based off of the original edition which I´m pretty sure leaves these factors out of its analysis.

Edit: Also, prices went up when it came to importing slaves from East Africa, but a lot of the price increase came from the extra precautions that needed to be taken in the mid-late 19th century due to the slave trade being effectively illegal in the Americas. The British were even enforcing these prohibitions by inspecting ships suspected of slaving.