One one hand you do have a definite demographic impact. Population in Africa will be larger. However, some of those taken as slaves would have died anyway from disease, malnutrition, war or other problems. There is a reason before modern medicine why African population growth was slow. I don't think the population would be too much bigger - at least not enough to matter when it comes to resisting European colonization in the late 1800s. There is a Malthusian trap here.
Another thing to consider is that Africans don't sell other Africans to the Arabs and/or Europeans, they have considerably less to trade. Ivory, gold, and some other products certainly have value, but slaves are a huge part of the trade. That means less salt, less firearms, and other products and amenities the Africans do not produce on their own (or produce much less or at lower quality). This will mean those societies are also much poorer.
Another possibility is that Africa becomes less culturally developed. The extra trade value of slaves, and freedman returning home means the transmission of Islam to those societies are less. It will occur much more slowly, and thus things that were included as part of that cultural package will be much less (like literacy, law, books). Without a significant trade surplus from slavery, how much of that money is going to be spent on importing korans, patronizing Islamic scholars, and other cultural benefits? It'll be much less as you concentrate your lesser buying power on necessary economic benefits.
On the whole, I'd think Africa would be even less developed, but with only a modest increase in population.
If you don't have the large existing slave market in place when the Europeans start trading in the 1500s, it will impede the New World economy initially. However, it'll likely develop one to feed the beast. West African kings will just need to create one which means lots of wars and empire building. Giving European made guns to favored chiefs/kings will be a significant advantage. We may see Christianity become more established in West Africa to curry favor with Europeans since the presence of Islam will be much less significant and important to elites without the stronger ties IOTL.
Overall, I don't think much changes on the macro level long term.