I think this would be super helpful for browsing sell/trade for the most part as well as other forums. I understand that giving users the option to ignore the older sales/threads would create some problems for sellers who are trying to get noticed with constant bumping (I do it to), but it would make it so much easier for buyers to find new content and items for sale.
Outside of b/s/t, this function would be just as helpful in letting users see new content without having to sort through all the bumps.
Ideally, the date/post sorting function would work in four ways. Users could sort threads
oldest thread-newest thread
newest thread-oldest thread
or the same two options for most recent/ least recent post date. This would give more order to the existing bumping system, as well as giving users the option to see the newest content first.
Hope this isnt a repost, or I hope this function doesnt allready exist and ive never noticed.