The Steam Machine is actually a great prospect — we're just looking at it wrong
(www.xda-developers.com)
Little late reply here... It just occured to me, that there could be a solution to this voucher selling problem. If you purchase the Steam Machine on the Steam page with your Steam account, then you could get the money onto your account. So selling it wouldn't be possible. However, they could buy and sell items from the Steam item shop...