Sounds true to life. If I give the bank £100, and then ask for it back... they will charge me bankers fees and I will get less than £100. True, the longer it stays in the bank, the more interest it will accrue, but the banks interest is always less than the rate of inflation, so the bank will always profit from keeping my money safe. (haha "safe"... a bank... lol. That's a good one, eh?)
In PT, the Castle owner gets "tax" on every transaction, so if you buy something and sell it back receiving the same value you purchased it for, the Castle owner will get tax on that amount twice, generating money in game from nothing.
Okay, that's what fiat currency is anyway, but once BC owning clan figures this trick out, they will buy and sell "Gold Bar" items at the vendor over and over generating infinite money from nowhere very quickly. ^_^ (well, I would; wouldn't you?)
If this was the case with all items, the same could be done by anyone.
If you want to make this "Gold Bar" idea work, it needs to be "tax exempt", and you are going to have to locate the vendor code and add a selection for "Gold Bar" transactions which bypasses all of the normal trade taxation system.
I haven't looked into NPC trading code to that extent, and this involves adding considerable logic, rather than simply changing existing logic which would, in one way or the other, break the monetary economics to the point where gold is worthless and infinite to all players. That may be "fun" but it will also skip a lot of game progression which is the reason many people play.
Just a fair warning. ^_^
