While it makes sense for discussion threads to be displayed in chronological order, in the Shopping section it makes sense to show newest post on top since we are typically interested in the newest deal and last month's specials are irrelevant to determining that.
If you like the idea, I'm willing to research phpbb modules to see if it can be done.
EDIT: Items for sale by non-vendor members would not benefit from this treatment but perhaps the posting order can be the thread-initiator's option.