Download old version of Mac OSX legally as Apple Developer?
Log into your account and navigate to the summary account overview page:


https://developer.apple.com/account/#/overview/


Select Downloads
Scroll the page so you can see additional downloads at the bottom
Uncheck all but OS X category
Search for 10.9 and grab the combo you want.



At that point, open a case with developer support and ask them to send you a redemption code for 10.9 for your Mac App Store account.