You still have to buy initial credit then minus one time activation fee before you can use your coupon. you cannot use your old account to redeem it.
Probably because it is hard to get banned I don't think there is a problem opening a new account and using it though.
If you have a current Adwords account you can "cancel" it and then open a new one and use the voucher. That is ok per a chat I had with a Google support rep. Having multiple Adwords accounts is not allowed.
A lot of people are selling without any fear and google is not banning anyone for selling. Ebay and DP vouchers section is full of such sales. You can get $10 for it easily if it is worldwide. Read the cover email to know whether is it worldwide or not. If it is only US/Canada accepted it can still sell for above $5
I suspect if you keep on opening new accounts, use the credit, then close it (to re-open another one) Google will catch on. As far as transferring credits I imagine Google has a way to track who the original owner is. Forget trying to sell the credit - just use it if you can. Else forget about it and just try to make a biz out of selling your services/goods on Adwords.