I guess they work out what they sold the last one for and go for similar. As most sales are based on an offer price which is then haggled down to a selling price the trade gets good data on what stuff will fetch. Supply and demand is the main factor I would imagine - when a new model is three years old and the ex-fleet stuff starts appearing in numbers then the prices tend to dip for example. Also a new model (e.g. new Range Rover) causes owners of the previous to upgrade, again flooding (to some degree) the market.
Neither is an issue for the Defender, which has been incrementally upgraded since the early eighties (or the late forties some would argue...). Defenders (particularly older ones) are not as subject to the usual concerns about age, colour etc, but far more on condition and specification.
Standard business practice for anyone who wants to get rich....
Tim Hobbs
'58 Series 2 '77 101FC Ambulance '95 Discovery V8i