I have seen many unscrupulous sellers selling various blizzard store items for stupid marked up prices.
These include Blizzard Authenticators which I have seen for £14, which are £6ish from the blizzard store which includes free postage. Authenticators are great for keeping your account safe.
The pets, like Lil KT I have seen going for £15 + on ebay, which again is marked up very high when they are £9 on the blizzard store.
Blizzard accepts all major credit and debit cards similar to what paypal accept so theres no real reason to be ripped off by these sellers trying to make a quick buck.
Enjoy your gaming!