When you buy an item you only buy a license to use it, not to redistribute it freely or for profit. The creator is not selling you the item they created but rather a permission slip to use it. I believe many software companies operate in the same way. After all, just because I've bought Poser doesn't give me the right to reverse engineer it and sell it as 'Hawke's Posing Program'. Where secondhand software is concerned, unless you transfer the license of the software to your name after you buy it then you technically have no right to use it - whoever owns the original license owns the right to use that particular piece of software.