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...