I volunteer all of the time and I'm assuming that they did not staple the conditions of the voucher to the back of each one that you received. I haven't received it most of the time.
The rules state that you cannot sell it.
I wouldn't risk selling it on craigslist..but I would not have a problem "giving" a family member/friend one if it is possible to pass them on to others. I keep them all for me, so I'm unsure if you are able to give them away.