electronics depreciate rapidly in price. some use 10-30% per 6 months depreciation i would say is normal depending on A) quality of the item and B) how in demand it is. of course such figures are only estimates. it looks like that tv is around 2012 era. if it is 4 years old then perhaps 10-20% of its cost new is a starting point and work down from there depending on condition.
a few things you have working against you..
- its older. people are not willing to pay much as it could fail at any time as there are no warranties
- its heavy. new models can be picked up by one person easily so a heavy model is not sought after.
- new low end brands are cheap. when you can buy a vizio for a few hundred dollars or even a cheap non smart sony for five or six that makes used tvs almost worthless unless they are top of the line.
in short.. you're going to lose your butt on electronics no matter what.
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.