the usual reason the devs would not want this to happen, is to prevent players for accesing event stuff that is binded... i mean, they could fix that by making a Red binding icon, to make some items imposible to unbind in first place (for example the prismatic cupcake, genuine halloween masks, kataclism shield, obsidian shield, tortodrone heads, etc [Not counting items from the hallow prize box and anniversary price box])
Your idea is pretty good, and it will give more life to the commerce between players even beyond the auctions (there is even a page for listing and coordinate trades between users, this will give it a lot more of life)