The second question is why, if he was a slave, he was able to buy slaves. The answer is that the first people bought as slaves in the English colonies were treated as indentured servants, not slaves, and therefore freed after seven years.
Also, “stolen” may not be the precise word. Most slaves acquired by Europeans were bought from Africans. This is not to justify African slavery, of course, but it is to say the myth that Europeans raided Africa is generally false. Europeans took advantage of an existing market. And that’s why several African countries have apologized for their part in the slave trade.