I noticed people selling their jetstar vouchers, when I look at the voucher I have it shows the following terms;
https://puu.sh/KI3PM/4f616360c5.png
Do they actually check that the person named on a voucher is named on the new booking?
Or do I just have a dud credit voucher?

It won't work if you are not the passenger. If you try to then you will get this error message.
It won't work even if you put your name as the contact person.