I would probably donate. I passed along my nursing tanks and bras to friends who had kids after me. They've been through me, friend A, me again, Friend B and now back to friend A who just had her 3rd kid. I like knowing they got some good use out of them and lasted so long.
Maybe this is me, but I'd feel weird wearing someone else's used intimate clothing (even bras). I would only sell or donate if they were new with tags still on. But that's just me.
ETA: If a very close friend or my sister wanted my used nursing bras, I'd probably just give them away.