Instead of having to remove and relist buy/sell orders on the trading post, would it not be better to allow an option to modify the price?
Since the trading post is playing the +/- 1 copper game anyway it would be a lot easier to just modify the price instead of having to relist everything.
In addition to this, the difference in modified price could be taxed at the same 5% rate instead of having to tax the original price you put the order up for again.