Trading Post Idea
Sale items: I don’t see that there’s a problem having items up for two weeks or longer. I’ve listed some stuff that I don’t expect to be worth much until October; I’m content to wait.
But there’s a bigger issue with automatically setting prices: this has unexpected impacts on the economy and can be manipulated by shrewd traders: anyone can hope to get lesser prices by simply listing an item repeatedly at a discount rate; those who are wealthy will gain from this and those who have less will not.
Instead, ANet could make it easier to relist an item at a lower price. Currently, you have to remove the listing and repost; they could add an “edit” button instead (same listing fee though).
Trade Flipping: you already cannot post an offer for less than 1c above vendor value; most of those listings are there because they were entered before that feature was added. Perhaps they could be removed by ANet.
However, there are a lot of people selling items for more than vendor value, e.g. selling Grand Master Training manuals for 2.5-3g (although they are available in every city for 2g). I think this is people trying to take advantage of other people’s ignorance, but I don’t think ANet needs to act to prevent it. People need to learn to check the wiki and the TP before listing a buy or sell offer.