Traveler checks are becoming a rarity, still safer than carrying cash, but in most countries in Africa they are very difficult to exchange. This is not the case in South Africa, where all major banks exchange traveler checks and credit or debit cards are widely accepted (some remote petrol stations might be an exception). In other countries, cards are well accepted only in tourist establishments. Avoid using foreign currency for any everyday transaction, you will always lose money and in some countries it is illegal. Always exchange currency at accredited institutions and keep proof of transaction until the end of your trip.