However, it appears you can give them away, by selling a blank envelope, or a drink coupon book, and including them for free. eBay doesn't end my auction if I do that. eBay did tell me that as long as I don't do any of the booking, they won't stop my auction (but I do, so that's why selling the envelope or drink coupons is better), but SW may send in a vero, and then they will stop it.
I've got two one way vouchers to sell again, and that is the only way I have been able to sell them on eBay in the past. I think I will try Craig's list this time, although I'm more uneasy about using that way. I do prefer though, that I can go the money order route and save the eBay and PP fees.
For some reason, Craig's List doesn't seem to have a problem with it, no matter how you list them, but SW does say they can't be sold.
I know nothing about the rules for the credits, but I have seen those auctions get pulled as well.