WI there is a significant overland transportation of European slaves over Sahara?
A lot of Europeans were enslaved by Barbary Pirates. WI the kingdoms across Sahara had a significant demand for luxury goods being white slaves, and bought them from Barbary?
Well, regardless the medical issues that other posters mentioned...
Barbary slavery of Europeans was really directed to North African needs (production and ransom), and while a sub-saharian trade is thinkable there's a reason why sub-saharian trade declined in western Africa : European trade having bypassed traditional trade roads and sattelized/raided old empires and chiefdoms, there was not as much incitative for Arabo-Berbers to put as much focus on them. (And the lack of slaves from Guinea provoked itself a need for European slaves)
Basically, European dominated Guinea means less likely ressources for a trans-saharian slave trade on a significant scale AND a probable European backleash witnessing it.
At the contrary, Arabo-Berber trade remaining the dominant one on western Africa, means less need for European slaves.
Altough you can think of a TL where western Europeans are down, where Russian slaves are still largely present in North Africa, around 2 millions, and see some of them on Africano-Islamic empires, I'm not too sure it would fill the OP, as in "widespread", even less on the historical triangular trade.