I think I already know the answer to this question, but...

[sgtJARHEAD]

Is it impossible to get back rare items after selling them? (i.e. Goblin Armor)

If not I may just have to load up an old file (I still may not be able to get it all back).

[TheSorrow]

As far as i know, yeah, it's impossible to get them back again.There are a FEW rare items that you can get by DDing normal monsters (very rare), so at least you can recover those if you sold them, but items that are gotten with special enemies, areas or events (like the Goblin equipment), only once.
You shouldn't worry much about them all though, some are only useful at some part of the game and later they'll just be rotting in your Elf's Haven thing, and there are ALOT of rare items to worry about storing them all. You won't need them later for the item completition (rare items are not included), and if you're worrying about not getting the full Goblin collection....it's not really needed to continue the quests in the following games nor you get anything for keeping them, and the summons aren't so great either :/