The value is whatever the buyer is willing to give you. If it is just excess and you don't have any plans for it then it is a lot easier to let it go then if you are planning on using it. Thats a lotta dust my friend.
My experience is that if someone from work wants to buy something that you did not overtly advertise to sell, then they are genuinely interested and will be more than fair on what they pay. If you just want to get rid of it, let him know of a general market estimate and they will pay that.