The boring, economist answer to this is it depends. There are a lot of factors that would affect this question, including tariffs and quotas, infrastructure, how different regulatory regimes are, whether two countries or regions speak the same language, etc. Trade volumes are largely a function of the size of any two given countries' GDP and the distance between them.
Oil is Nigeria's main export, so it may be an energy exporter to Mali assuming Nigerian history is relatively unaffected in this ATL history (Biafran secession fails, etc.). A surprisingly low amount of African trade patterns involve neighboring African countries. A lot of exports are still natural resource exports to the former metropole, while imports are port and rail infrastructure to get the resource to the coast. Much of the railroads and Ports built by the European powers, as well as the new Chinese infrastructure investments follow this general pattern.
Africa is steadily moving away from this pattern by building infrastructure that actually ties countries together, the shift is also visible in capital cities being moved from the coast to more centrally located spots (Abuja, Dodoma, etc.). The African Union is trying to facilitate greater intra-African trade ties by removing tariff and non-tariff trade barriers through a Continental Free Trade Agreement and enabling migration through a common AU passport.
As long as ATL Mali's government avoids grand top-down scheme and the urge to micromanage the economy the way that most mid-century African leaders did, it will do relatively well. If a state tries to do everything, it generally ends up doing most things poorly and enables massive levels of graft and nepotism. When a state is limited to the provision of public goods like education, healthcare, infrastructure, defense, etc, it can specialize and do a relatively decent job at those few things.