There are many reasons why defenders have a high secondhand value, besides what people have written above:
They are very easy to part out, and with ebay and the like, secondhand parts have specific and high values, making the floor of the market for complete cars specific (and high) too.
The second hand market for defenders is a minefield, the huge popularity makes finding a good, solid and reliable defender for little money near impossible (as if it is truly solid and reliable, very few land rover owners will sell!). This allows the cream of the crop to rise in value significantly. Then there are sellers that jump on this bandwagon advertising their car as the cream of the crop, with high prices but actually shoddy underneath. It seams like the market went through this and then the real, actually solid cars went up in value even further!