Hubbe,
What price point did you see the BL5's for?
Sometimes you never know what financial position is in and if it's all borrowed money, sometimes you are forced to sell.
I've had good experience recently buying BL4000's from another guy here in Melbourne. The smart way would be to contact the seller and ask them what is the type number, then you can research their age, etc.
As they are not on the market that often, it is in this case the market that sets the price. I don't think it's like a car where there is a nominated depreciation (which may cause some debate), but in the end the market sets the price ultimately.