Alright, so the concept is pretty basic. Searched though and didn't see anything about this.
1. Go to any retail store that sells WoW stuff. I prefer Best Buy cause they're 1337.
2. Purchase a World of Warcraft Battle Chest. I got mine for $30 USD from Best Buy. This way, you get a WoW key, and a BC key. If you buy them individually, I think they're $20 each. So go with the battlechest and save yourself $10 for now =)
Note: The sticker shows $40 because that's what the price was before. The receipt is $30 though.
3. Go home, or anywhere else I guess after you leave the store. There is a clear plastic, circle seal on both the top and bottom of the box. DO NOT just cut through it with your keys or something like I used to do. Instead, take a knife, or anything else thin, to carefully remove the seal on one one side so you can open the box.
4. Pull out the cardboard tray. There will likely be a product catalog, the guide for WoW/BC, 2 trial key flyers, and the 2 CD sleeves for WoW/BC.
5. In the past, they used to give you like 5 CD's to install the game on. But now I guess they compiled everything down to 1 CD for WoW, and 1 CD for BC. Since it's only one disk, they have another clear plastic seal over the opening to the sleeve to hold the disk in. LEAVE IT.
The CD Key is right on the front of the sleeve. Write it down, or enter it in to create the account right away.
6. Now. Take the trial flyers too if you desire, but it would probably be less suspicious if you just left them. (They're pretty worthless, but I guess you can write down those keys too.) Put the sleeves back in their place with the rest of the stuff, and slide the cardboard tray back into the box. Reapply the clear seal over the top of the opening and Ta-Da!
7. Take it back to where you bought it from. I know Best Buy has a 30 day return policy, so bring your receipt and take it to the returns desk and when they ask you why you're returning it, just say something simple like "I was buying it as a gift for my friend's birthday, but his parents already bought him it." Anything that says 'Here, I want my money back.' while not saying 'Here, I opened it, and didn't want it anymore, I want my money back.'
(I know it's dark >.< but it's a receipt)
Don't be nervous! Don't freak out if they ask you why you're returning it. Know what you're going to say ahead of time, and act like that IS what happened.
8. Leave the store. Congrats, you just got your $30 back, and brand new CD Keys! Whether you sell them off, or use them yourself is up to you. Selling them may be a better option, since when the retail store finally does sell that battle chest again, the poor guy who tries to enter the keys is probably going to complain that they are taken, and the account with the keys might be locked.
I would advise not repeating this more than a few times per store. If you live near a large city where you can get to multiple retail stores that sell the battlechest, this could be VERY profitable for you.
Enjoy your free CD Keys!