I've also bought used gpu's @ absurdly low prices on ebay. In which case the time to 100% roi is vastly smaller than buying the newest line of gpu's.
Sure, but if you wanted to get cheaper GPUs via ebay, you would just buy a new one yourself, and then sell it yourself later on ebay.
That way, when you buy your card new, you know that it actually works, and you actually have a warranty.
I've bought 2 old computers on ebay before, and both broke as soon as I first switched them on. So, if the price drop was huge, and the ebay seller promised you a warranty, then maybe you have a point, but otherwise I'd rather buy new.