The same thing happened to me in reverse last week. I ordered something from Amazon and it arrived in an Ebay box. I can only presume people who sell on Ebay also sell on Amazon. Likewise, it's not beyond belief that Amazon itself advertises and sells on Ebay under different business names. If you bought from Ebay and the item is not wanted or faulty then you will return it to Ebay, regardless of the fact Amazon may have dispatched it.