Zed prices never seem to make sense. There's "what they are worth", and "what people sell them for", with very little correlation between them.
You see people slashing the price on their HR for a fast sale and selling them for £4k when they could easily get £9k.
Then you get people who manage to sell their DE for £10k+
It's madness, but I think a lot of it it comes down to right place, right time (though condition and quality pays a big part).