Initially, Britain sent its criminals to work in the colonies. This emptied their jails, decreased costs, and supplied the colonies with labor. Indentured servitude, unlike slavery or forced emigration was based upon personal choice, but many people had no money for the initial voyage. Thus, merchants, landowners, ect., would pay for these people to come to America in exchange for seven years of servitude.
The first slaves came to America in 1619, arriving in Jamestown. This initial form of trade, like the prisoners from Britain, was based upon the African tribes selling their criminals.... the unwanted into slavery. As time went on, it became a lucrative business that included kidnapping and enforced slavery.
Lastly, you ask about the decline of the Native American population in the East. The primary reason during this period for the decline was disease. Both the colonists and the Native Americans came into contact with unknown diseases..... illnesses connected to diseases they'd never been exposed to before. The environment on slaves ship was perfect for breeding small pox and yellow fever. Small pox was one of the largest causes of death to the Native American and slave populations.