CNC use only I'm afraid. The BT part refers to the tool changer type the holder is suitable for. (Or vica versa) Value, yes they have some value, not what people on ebay think they are worth though a lot of the time, hence why they don't sell. The problem is cheap holders are now available and at very silly knock down prices and they are very, very good for the price you pay, They are as good as expensive branded gear in accuracy and usability. This is where is goes wrong for the second hand market because new are so cheap.
The only way they may be worth a little more is is they are a decent brand and rated and balanced for higher rpm, these are more expensive than the generic 8k spindle tooling. The next standard rating is 12k spindle approx. The slight problem where sellers fall down is because they don't know what they are looking at with these ratings and cant advertise it as such because they never appear to know what they are looking at.
The condition of the shank is another governing factor to effect value, No one wants to put a shabby bad condition shank in expensive high rpm machine tools.
I look at a lot of this gear on the bay having a BT40 machining centre, so much never sells unless its the right money and good condition.