People are buying up Gtx 1080's like a fat kid in a sweet shop, anywhere from the pretty standard rebranded vendor cards like the Asus, all the way up to the heavily modified versions, makes sense that anywhere selling these cards would increase the prices to capitalise on this, as for buying second hand, the answer is you won't, if you pick one up on ebay theres no way of telling if this is a mining card being used constantly for however long or if it's just a reasonably used card, if it's not a massive requirement to jump up to gpu rendering now, i'd wait on the next generation cards, ( which i have a feeling will be quite soon) once these hit it should drop the prices of the 10 series to something atleast half reasonable