Theres a few things i do to judge the market value of a motor im looking to buy,first its do a national search on auto trader web,put in the age & max miles you want & the search will show loads of cars,that will give you an average,also do a detailed advanced search on ebay of completed items,the ones in the green are cars that have sold,this shows what the cars are making,the last thing is talk to a helpfull dealer & ask him what they pay for the model your after in a trade in,the trade in figure will be about the lowest your pay for 1 but it gives you a base line,the closer you get to the baseline the better deal you get,i would not buy from a main dealer unless the car still has manufacture warranty ie under 3 years old but that just me,but like you said i car is worth what buyer is willing to pay for it,good luck with the search