I've bought a couple of pets through eBay lately and so far, my experiences have only been positive
That being said, I also have my eBay-related pet peeves. For instance, I find those sellers a little irritating that describe their collector's editions as BRAND NEW or FACTORY SEALED in the titles only to clarify in the description (sometimes in rather unobtrusive places) that they're only selling the extras. Or those sellers that don't describe (as in using words) what they're putting up for auction and just state something like "you get what's in the pictures".
Indeed. Those are quite annoying ^^ I've bought a lot of stuff from eBay, not only for myself, but for the raffles and my friends as well. I've yet to be scammed, but you do need to make sure you read the entire description. Some will say "New" if the codes are used, others will have a really cheap item but then charge 6-7 times the items price in shipping. Some will list that the game is included, but by this they mean the CD itself.
It can be really confusing
I stay away from the ones that state they don't know whether the code has been used or not.
It is possible, though, to contact Blizzard and ask them to confirm whether a code is still valid. I've done this multiple times. Simply ask the seller to do this and if they refuse then don't buy from them.