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 (http://www.whatsupaddis.com/). 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