no you dont need to buy a new copy.. after u upgrade the trial to full account, you can still play it like you have bought a new copy of WoW.. jus that you dont have the installation cd.. but you can use the link provided by blizzard to download the whole WoW installer.. =) it's cheaper that way..

i think Echhtros mean he used the 2nd hand wow copy to "REFER TO FRIEND" account, that mean he used the account to refer himself a 10-day-free-trial, registered under him name.. in that case, he can upgrade that trial account to full using credit card without buying a new copy of wow.. cd key is provided during the upgrading process..

Cause i dont think he mean he get a 2nd 10 day trial lol.. that doesn't make any sense cuz it's 2nd i think must be over than 10 days?? =D