Steve, you can actually get boxes from the usps for free. Go to their website, and you can "order" priority mail boxes at no cost (any size to boot). Just cut a few slits, turn them inside out, re-tape, and you have a plain ol cardboard box to send stuff in. I get stuff off ebay like that all the time.
The only warning is that once you have "ordered" stuff from the usps, you won't stop getting new shipments (including stuff you didn't even ask for). You gotta look under the business section of the website.
Here's a link:
http://shop.usps.com/cgi-bin/vsbv/po.../Priority+Mail