Showing all matches has been tried out in the past and the biggest issue was page load times. There are members with 50,000-100,000 items on their wantlist and the page would take several minutes to load as it matched that many records in the background and generated the report.
Collection matching was almost actually removed as an option because it was bogging down the site as a whole, but then breaking it down by set helped make it a manageable prospect. Worst case scenario, someone has a whole set on their wantlist and is matching it against someone else with the whole set and there are only a few hundred entries being compared as opposed to someone matching the 50,000 items on their for sale / trade list to someone else with 100,000 items on their wantlist.
If everyone had small wantlists and for / sale lists, then it would be no problem, but that’s not realistic and whatever solution is in place has to be able to work for all members regardless of list sizes without affecting other aspects of the site.
Perhaps if after adding items to the transaction it didn’t bring you all the way back to the transaction, but brought you to the list of sets, this would help speed things up?
Vote for and track new site features in the Feature Tracker.