there are 2 issues here
1. when purchasing from a merchant ie fuel buy 1 item for 6 copper
6 copper is removed from your player coins, but the chat window display say you bought for 0 copper.

2. when purchasing a qty of items ie 10 fuel, only the price of 1 fueal is deducted from your account

the problem has been identified and a patch is in the works

I have committed a fix for this SVN it is included in SVN revision # 2798