Can I sell discontinued printed packaging? How?

I wouldn't recommend it unless it's really, really generic and there are no trademarks, logos, or copyrighted materials anywhere on the packaging.

I assume that the discontinued printed packaging includes a trademark of the original customer - a logo or word or both. You can sell it - as long as you make it clear that you are a re-seller - selling discontinued product - and your have no affiliation with the original customer. You are selling "junk" at a discounted price - not anything else. Be careful and you should be okay.