For rapidly evolving tech then what you paid for it is totally and completely irrelevant to what it's worth.
Find the price of what a technically equivalent new item would cost and then discount from that for the used status (generally at least 50%) IMOP.
That means that to value a used GTX Titan GPU ($999 in 2013), Find the price of it's current equivalent (GTX 1060 at $249 or RX470 at $185 per TH GPU Hierarchy) and then reduce that price by half! Half may not even be enough of a discount for TVs and graphics cards because of the IO and standards changes that make a newer "low end" version much more attractive than powerful but older tech.