From where I sit the question is reverse. Where can us gem and mineral collectors find genuine and trustworthy sellers?
It is all about trust and too many sellers have proved deceitful and even damn-right thieves. This affects other genuine sellers in those countries as buyers refuse to part with cash. Your best bet is to contact someone within Malawi who buys rough stones to sell on in the market. You won't get the price you would like but at least you will get some cash for your efforts.
Start small and become trusted. Once there is trust in what you sell and the way you do business and you will find that buyer, through word of mouth and recommendation, will come and find you.