Is there a standard rate exchanges for buying and selling in Africa? Some countries have irregular and unfair rates . Is there even a regulatory body to moderate things legally?
It's most likely due to the fact that a lot of African currencies are illiquid and/or have capital controls, which means that the market prices for BTC and other liquid, international cryptocurrencies will be significantly higher in comparison with the free markets elsewhere in the world. This has happened before elsewhere, e.g. with INR, KRW, and CNY.
There is no one that moderates BTC price, nor can any single entity do so given its decentralization. It's essentially something that is built in, so that no central bank can manipulate BTC's supply or whatnot, even if that does lead to short term volatility. So no, there is no legal regulation for any exchange in the world to uphold a rate that is close to the going market rates, because it's simply not up to them to control.
My suggestion to you is if you can, convert your funds to USD or other forex first, then buy cryptos on the open market. Though, the premise for that is that you need to have a means to convert your local currency to USD without significant markups/spreads.