How much are you looking for and what is your price? There are many people selling Starbucks Cards here so you should take a look around.
There's really only one who has been around consistently for months with hardly any unresolved issues. I don't want to sound like a shill so I won't drop a name but it's easy enough to find. There probably hasn't been more than one other seller I can think of who's been around for more that 2 weeks that hasn't suddenly disappeared under a thndercloud of complaints.
