Buganda didn't really get into the slave trade until Arab colonialism broke into the Bugandan markets after the 1840s, when Buganda was already established and growing. Even then, the slave trade was primarily on the edge of the economy, in order to exchange for guns. Most of the economy was built on iron, salt, and food trade. See [1], specifically chapter 3, on the formation of monarchies.
[1] https://www.goodreads.com/book/show/277585.The_Great_Lakes_o...