Ben has an Nvidia GTX770. One generation old and pushing two years old. Damn fine card to say the least. New they were around $450.
Nvidia just introduced the Nvidia GTX970. A solid bump in performance from the 770. That said not enough to pay $340 which is the going price for the 970.
Now, since Nvidia is no longer making the770 anymore the folks who need one (so they can put two together) will pay more for a used unit than you might think.
So. Say a used 770 was going for $210, now they might be pushing $260. Thus Ben sells his two year old 770 for $260 on Craigslist. Then gets a brand new 970 for $330. He gets a new card of the latest build for $70. Seems like an easy decision.
Please note I haven't researched the numbers. Those are likely fairly close however Ben would have to do a quick search to see what is what and do the math. I have done this myself in the past and it is a fairly common practice in the gaming group I socialize with.