Hi Jenny,
Cars are quite expensive here - there is a hefty duty on any imported car. The price posted for the Nissan is a typical price for a duty-free car. You can usually count on recouping that money when you sell it - cars do not lose value here like they do in the US and the turnover in foreigners means that there is a market for duty-free. Most Ethiopians, if they can afford a car, have a very beat-up Toyota Corolla, or a tiny, new, Ethiopian-made car. I am guessing you will not have duty-free, so you will have to either buy a duty-paid car or a car that is more than 10 years old (duty expires after 10 years).
There are some classifieds in the What's Up Addis guide (). The website is not particularly updated, but has some car ads. Car ads are posted at the big supermarkets around town.
If you are traveling around the country a lot, you will probably want a 4x4...the roads are terrible outside the city, and many places in the city too, but in the city you can manage with a Corolla. Going long distances outside the city in a sedan would not be a good idea.
Gas is about $3.50/gallon right now.
Good luck!
Jess