I foolishly sold almost all of the special/unique loot you get from completing the campaign levels the first time. I didn't realize you'd be able to go back through and re-run all the levels with challenges, so I'm feeling pretty dumb now since some of the stuff was pretty decent. I know there's lots of great loot in multiplayer but I was wondering if there's a way to get these items back or get them again? The stores have refreshed and no longer have them for buyback.
There is only one undroppable item, a level 30 treasure called the White Star Diamond. All other items, including first-time drops can possibly be dropped from MP and SP chests, and most (excepting lvl17-19 commons and uncommons) will also appear in one of the shops.
There is no such thing as special/unique items in regular adventures unless an event is running with special items like halloween or December events. Any item from the basic set or from the Attack of the Artifacts expansion can be dropped at any time if you run adventures around the level of the item and you get lucky enough to get that specific item. Now it's sad indeed that you sold everything you didn't use, gonna be complicated for you to complete quests like equip only items with drawbacks ones. I suggest you to level up another team up to 20 (or several other teams if needed, I have personnaly 3 characters of every class/race combination possible for a total of 27 level 20 toons) in order to get back some stuff. Having different kind of characters can become handy, especially if they make some new modules again. You can never know what you will need ^^