Do the 5000 items need to be unique? I know the '1000 gallery items' are unique, but what about the shop items?

Nope you can practically have 45K of the same item :)

They do not need to be unique for the Super Shop achievements... ^_^

haha thanks, that's awesome. I have so many duplicates, I'm glad I haven't sold them all off yet! thanks again!

You can get all the shop item's that are wearable's which will help you get your goal.