Try avoid buying a car from agents or from shops such as the ones located on Salwa Road. You wll end up paying extra for nothing. Even genuine owners of vehicles usually advertise their prices for their cars on the higher side. There always exists a margin for negotiation. Many factors come into play when assessing the value of a second-hand car. The model, the kilometer it has done, overall condition from inside and outside, any unusual noise while driving, are to name a few. Added to this is one's knowledge of what is the current value of the vehicle in the market. As britexpat has very rightly said "there is no exact science."