Explanations about the car options for an expat (Tunis)

Hello,
I've just arrived in Tunis and will be staying around 2 years. I would be interested in buying a second hand car, possibly cheap (I will have another "main" car provided by my company, but only in a few months). But before that, I would love someone to explain me a few things about car registration in Tunisia :
- RS plates : I understand someone with a RS plate cannot sell his car because he has not paid import taxes. But then, what happens if I want to buy a car with RS plates, like the one Feryal Jehane is selling on another Internations thread?
- PAT plates :what is this, and how does it work as compared to RS?

Also looking for some furniture.
thanks for your feedback
cheers
Laurent