I bought a Bose off ebay about a year and a half ago. It had very little use, and ran like a top. I paid about two thirds of what it would have cost to buy one new.
Try Marvgolden.com. They've always had very copetitive prices, and I've bought several headsets there.
Many times flight schools will have bulletin boards with notices of things for sale; headsets come up all the time.