here is exactly the same problem with selling and buying food, drinks and everything including the hearthstone. I had deleted the cache and logs, shutdown the computer before restarting windows, wow and HB. The first thing the toon does is go by every vendor in gadgetzan and ends up selling everything. Every time it buys food, that is about 5g at this level. That is a lot of gold if you multiply it by an unknown number of times. Not to mention the loss of time.
edit: my toon has lost more than 50g in less than 10mn!
edit: my toon has lost more than 50g in less than 10mn!