Yes, it's forbidden as vouchers are for personal use. A better question though is if it's possible for Google to find out that you've sold a voucher As for buying vouchers and using them, be VERY careful. Google has started banning lots of publishers that have seemingly created multiple accounts to get the maximum out of their vouchers. And the techniques they are using to determine such cases are excellent.
In theory they could certainly sue you for breach of contract. I doubt they ever would though for this sort of thing.
They CAN but if it's not a massive amount then they won't bother. Nobody is going to get sued over $50.