There are several ways one can buy a car in Sierra Leone, be it new or used. One can contact car dealers whose points of sales are popularly located along major roads in the city and in the provinces. People could opt to sell their cars when they see the need.
In Freetown, just strolling along Pa Demba Road gives several of those options for used cars. Circular Road, Big Water Loo street, Wilkinson Road, Bai Bureh Road (in the Eat End of Freetown) etc., all offer other centres as well. Dad's Car centre, Lumley, Freetown, sells brand new cars of all types.
In Bo, the southern capital, Tikonko and Sewa Roads have car sales centres. In Kenema, the eastern capital, Hangha Road (just around the Police Barracks) has a used car sales centre.
After purchasing an unlicensed car in Sierra Leone, the next step is documenting it for road use. The Sierra Leone Roads Safety Authority (SLRSA) handles that. It can cost about USD 120 for private license and up to USD 150 for non-heavy duty commercial license. The licensing process is usually boringly long and in many cases requires informal approaches to get one fast.
Sometimes when licenses may not be available (I understand they are done in Nigeria for the SLRSA), the Authority can grant a three week unlicensed vehicle pass to allow it's road usership until one gets his or her own. It could cost between USD 30 to 50, considering the informal but known and acceptable routines.
If one buys an already licensed car, the only procedure then remains the transfer of ownership through the Authority. It does not normally take long, can be done in a few hours depending on the availability of the Authority's official in charge. But I don't know if it involves any "payment for forms" though I will not surprised if it does.
Please note that all these costs stated here could vary in time as price control is one war of its own here. Kindly seek other people's advice and compare with mine before using this information.
I hope this is useful.