Your first sentence is bollocks. I'd be fascinated to hear if this ever happened but it definitely wasn't the norm.
So because some Africans were complicit in the slave trade that makes the whole thing OK and absolves Britain from blame?
Thast called a strawman. A debating fallacy used by children and clowns.
its the act of actually throwing a net over someone that is bad, not trading them like livestock
It was Africans doing that to each other...and then bringing them to the ports for Europeans etc to buy. Just like they had been enslaving each other for 1000's of years prior.