Yes, they have residual value. So you can ticket as many times as you want within that value.
Yes, they can be transferred, but no they cannot be sold. They require the person whose name the vouchers are issued in to be physically present when the ticket is issued, even if the ticket is for someone else.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada