I haven't bought extras of a boxed set myself, but I have:
1) seen broken sets on CM LE's. For example in May I saw the CM Holiday set quite a bit. But I had also read the CM store was selling these sets for $5 or $6 each.
2) I have traded multiple pins to get a boxed LE set.
I generally stay away from boxed sets unlss I really like them. I haven't some uo with a great way to display them. So they sit on the boxes on a shelf, in a plastic DS bag. (Not much point in having them really.)