Point being they couldn't purchase mass amounts of slaves. West African slaves primarily came from other conquered African states.Most Europeans didn't go in slave raids when they wanted to get slaves, most of them negotiated with the kingdoms in the area for slaves. The only exception are the Portuguese. So I guess the Russian and ottoman governments could sell some slaves to make an extra cash, since they were at war all the time.
Also, don't the boats already come from Europe? So it's easier to make a trip from London to st. Petersburg to new York than to go to west Africa
I don't know the exact logistics, but using London to New York is a bad example as thr majority of slaves were sent to South and Central America. Brazil is particular, which was Portuguese owned and across from the pond from Portuguese dominated West Africa. Much of the same goes for Spain. Ignoring the logistics. West African slaves would most certainly be less expensive than Russian serfs. Of course, the issue is that there was no widespread slave trade in Europe even before the discovery of the America's.