I've already made a bug report about it in-game, but I figure I'll post the bug here too.
I had some items that were up for quite some time and I wondered why they weren't selling, so I did a search for that item. It turned out that the one I put up wasn't showing. I re-added the item to the Exchange and then it showed up again on a search.
This got me thinking though so I also checked up on other older items, turns out none of them show up.
I think this bug is caused by a certain limit the Exchange has with handeling new items, where old items are pushed off the list and don't show up anymore, to make room for new items. Because of this old items will never show though, even if they're cheaper then their newer versions, since they can't be found anymore.