The Motor Trade Generally used to use 2 guides to buy and sell cars
They obviously buy on the cheapest and then quote the most exspensive when selling.
Parkers has allready been mentioned.
www.glass.co.uk
Link below is for the valuation page. But sadly costs £3.50 for the valuation one you have filled it all in, but on the way through does show all the options at the point of purchase, so may have some uses. :-/
http://www.glass.co.uk/app/plugin_uc/_src/ident.php?sess=UKucnc51dcae255415733c92bdc5b673291278158796&l=en&UCvtype=10&h=www.glass.co.uk&newsess=yes&premium=0
Parkers is a joke matey,the sort of wally who has come around my place armed with one of them gets shown the door pronto if they start up about what them things say,i have seen stuff thousands behind and infront and would never trust them even if they get it right.
i use CAP Guide that stands for Current Auto Price Guide and has been around decades. Its the little black book and even that gets it wrong same as Glasses.
if your a Account holding member of Manheim you can use CAP for free against the stock they hold.Bargain

the best way to know your stuff is to see what other people are selling stuff for,it was always autotrader before ebay.If you saw say a car in the trade for a grand and in the autotrader they were 2 grand its chances are theres a earner there.So knowing the market is the best way imo.
