- If you are buying the physical item (such as a TCG card), you use real money.While there are shades of grey for some transactions, it's likely any deviation from the above is a scam and participation in it will get you suspended/banned. Customer Service/a GM probably won't be able to restore any currency used in-game, and will probably remove the pet you purchased.
- If you are buying an already in-game BoE item (such as a TCG mount/pet that's IN THE GAME and BoE or the Guardian Cub that's IN THE GAME), you use gold.
- For virtual goods that are NOT YET REDEEMED/NOT IN THE GAME YET, such as the pets from the Blizzard PetStore, you use real money.
If it seems questionable and you're not sure if the sale is legit, just don't do it. As adorable and awesome as vanity pets are, they're not worth losing your account over. Besides, without that account, you can't even use the pet or have a collection! So just play it safe and use common sense/your best judgement. :)