I was farming red dust in the PvP area aiming for 85k to buy a mount.
I had 7k to go, handed in the last of it and then noticed I suddenly had 58k?
The only thing I think could have happened is that I had an item drop from a mob (knights box or something) that is also purchasable from the vendor for 30k.
It may be a bug in that clicking the item to claim it is charging the player as if they purchased it from the shop.