I've realized I know a fair amount about slavery in the US in general, but ended up stumped here. Basically, I'm asking about that one person (or group of people) who once found themselves in Africa and went, "Huh. We could put all these people on a ship, take them home and make them our slaves."
Who was it and when exactly (or as close to it as possible) was it? Was the idea initially accepted (everyone jumped on board and approved) or was there opposition in the early days? What did the international community at the time think about it?
I did a quick search and didn't find a question as specific as this one. If I missed it, my apologies.
I couldn't give a very in-depth explanation as this isn't my field of expertise, but I can tell you that slavery was a thing in the US since the very first day it was founded.
Slavery had been practiced in the US even when it was still a colony, so its no surprise that once she gained her independence it was already a long standing practice.
As evident during the Civil war, the southern colonies found the greatest use for slaves, using them to farm vast fields of tobacco and cotton. In many regions in the south, it wasn't uncommon for slaves to make up anywhere from 60% to 80% of the population. While all British colonies in America legalized slavery early on, the territory of Massachusetts was the first to do so in 1641.
By the outbreak of the Revolutionary war 20% of the population in al 13 colonies was of African-American decent.
The first slaves in America arrived in August of 1619 aboard the damaged White Lion, a Dutch ship commanded by Cpt. Colyn Jope. As stated before, slavery wasn't first legalized in the first colony till 1641, so the fact that these colonists openly accepted the 20 kidnaped (and illegal) Africans aboard in exchange for services, is testimony to the popularity of the idea.
Anyway, that's my two cents on the subject, I hope it helped.
The other posters here are exactly right, but to I'd like to add a bit to what they've been saying. So, slavery in the western hemisphere predates British colonization by a decent amount. The Spanish initially exploited the native population in the Caribbean and Latin America, but as they died out they decided that importing African slaves would help keep their plantations and mines going. Bartolome de las Casas is generally credited with kicking off African slavery in the new world, though historians still debate why exactly he endorsed it. After him, African slaves became a staple of Spanish and French colonies in the sixteenth and seventeenth centuries.
In what would be the continental US, it's pretty well established that the first slaves arrived in the colony of Virginia in 1619, brought by Dutch merchants. Slavery remained fairly low key in British North America for much of the seventeenth century, as the regular influx of indentured servants provided much of the same labor. Slavery became a very profitable industry in the Southern colonies because of the types of crops that they were growing, tobacco and rice most importantly. New England Puritans also had some slaves, though never on the same level for a couple of reasons. One, they were mostly growing subsistence crops. Two, the Puritan mindset was very family oriented and did not allow much room for outsiders. This is not to say that they did not have some slaves, but they were in much smaller numbers than in the South.
To get to your other questions, slavery was a pretty well established tradition in Spain, Portugal, and the Mediterranean, so there wasn't a whole lot of resistance at first. Slaves were used on the Canary islands and in sugar islands in the Mediterranean long before the New World was discovered. It's important to note, however, that this was not race based slavery. Exactly when people thought of race as a primary identifying category and when they started using it to justify slavery is still a historical debate. Personally, I think you can trace it to the Reconquista, but others have argued that it developed in the New World during the seventeenth century.
One of the first examples of opposition to slavery in general that I'm familiar with is Samuel Sewall who was a Puritan merchant in the last decades of the seventeenth century and the first decades of the eighteenth century. But, to be honest, not many people were listening to him at the time.
Sources: Joseph Miller, The Problem of Slavery as History, Kristen Block, Ordinary Lives in the Early Caribbean, David Boruchoff, Another Face of Empire, Ira Berlin, Many Thousands Gone, Judith Carney, Black Rice, Philip Curtin, Rise and Fall of the Plantation Complex, Edmund Morgan, American Slavery, American Freedom. I could go on since this has a huge historiography, but I'm tired.