1) Cash - this is the fastest and easiest method. bank transfers can be slow and create trust issues due to the timing of payment arrival therefore cash is ideal unless budget is very high.
how to: Bring USD or Euro with you (you are allowed to bring up to 10,000usd per person) then change them at the AFEX office (not the one at the airport its more expansive).
2) Credit card/Debit: this method is also easy and quick, however has commissions and can only work if you buy from a car dealer that has card payment (this eliminates many good cars from particular sellers)
3) Remitly - a decent service that can either provide quick bank transfers or send you cash (limited to 2,000,000 in most cases)
4) transferwise - easy wire transfers.
5) bank transfer - the slowest and most expansive.