From About African History:
This is not so, and punishments meted out could be very harsh.
From 1680 to 1795 an average of one slave was executed in Cape Town each month and the decaying corpses would be re-hung around town to act as a deterrent to other slaves.
See also here.
From the same site:
Slaves for the Trans-Atlantic slave trade were initially sourced in Senegambia and the Windward Coast.
Around 1650 the trade moved to west-central Africa (the Kingdom of the Kongo and neighbouring Angola).
For more details on this, read How Many Slaves Were Taken From Africa?
Comparing slavery in North America to elsewhere: here.
Timeline of slavery in the Cape Colony: here.