Well, even if you get good deals on used 280x and 7950's you still wont ROI at all... Save your time and go collecting bottles instead, that would actually be higher pay per hour 
Sorry if I sound negative but that is the sad reality of it. Now for people who just want to learn it does not matter that much what gpu you get your hands on, all amd from 7950 and later can mine all gpu coins but nvidia starting from 750ti can mine more power efficiently, bear in mind choice are limited then and if those coins dies then you are out of luck, amd might be safer choice then.
Sorry if I sound negative but that is the sad reality of it. Now for people who just want to learn it does not matter that much what gpu you get your hands on, all amd from 7950 and later can mine all gpu coins but nvidia starting from 750ti can mine more power efficiently, bear in mind choice are limited then and if those coins dies then you are out of luck, amd might be safer choice then.