I have never given just used items as gifts, but I frequently bring a small new gift (something you use up, like a collection of your favorite baby medicines, perhaps) and then bring the used items in a nice box (NOT wrapped).
I usually put the present with the other presents, and when I get a moment alone with the mother-to-be (or talk to her beforehand) I will say something like "I put together some of my very favorite stuff I used for my babies - take a look and if you don't need it, let me know and I'll take back whatever you don't want."
The other really important thing is to ONLY give someone your very nicest hand-me-downs. Don't use a pregnant friend as a chance to clean out stuff that should go in a garage sale. I love getting used items (I'm also a granola, like the other mom) but I do not appreciate having to sort through a large quantity of items.