It's not about Nigeria per se. It's about drawing the line. AVIC sold a lot of M60 in various poor places like Cambodia, Myanmar and a lot of them had incidents, many due to human errors, but the blame obviously comes on AVIC too.
The countries' population and economy shouldn't matter, only their track record and experience of actually maintaining and operating aircraft.
Maybe Nigeria is okay, maybe its not. But no need to rush. There's no benefit of selling these aircrafts at a cost (let's face it, it will require substantial subsidy in initial years to force someone to choose C919 over Boeing/Airbus) when these countries go out there and crash it as well. One incident and that will be featuring in the news with every C919 report for a decade. Not worth it.
For aviation, approach should be from top down. Go into premium markets first, obviously depending on certification and other things.
Chinese domestic market is more than enough to sustain C919 (and then C929) for over a decade. Then head out to friendly countries which have good rep --> Malaysia, Indonesia, Russia for instance.