When purchasing a used World Of Warcraft Game make sure the seller is going to include the account with it!.
Why?. Because the key can only be used ONCE with ONE account. Once an account has been created the key cannot be used again!. If you try to enter the key all you will get is "This authentication key has already been used. Please contact billing@blizzard.com".
You can purchase keys online, However they costs just as much as it would if you were buy the game brand new.
Now dont get me wrong, buying a used game with the account included can save time & money!. If not then expect to do some hardcore gaming, as to get to the next lvl can take 6 hours & doesn't really open up till lvl 40+.
At the time of writing this guide this i had been playing for a total of 3 Days & 19 Hours & 19 Mins (Totaling 91 hours) & i am only on lvl 22. It is a great game but there is a lot of walking to places & you can spend hours uncovering the map.

About Gold Guides
There are pleny of gold guides to be had on & off eBay, but they are not much use to begginers as you need to be at least lvl 40. Also most are free in the public domain & a lot are just coppies taken from the website guides. Fair enough if you want a copy on your PC, but personaly i would rather save money & visit the various sites to view guides.