Nobody really knows, usually prices of TV's which are around that size i would say around 200$-300$? Im not really sure, since its 2011 model, anything can go wrong with it, rather buy new TV with warranty, usual lifespan of TV's new generation LCD is around 10 years or soo, they might last longer or less, nobody knows.
If it is LG, some of them tell you how long it's been used for in the settings/information somewhere. http/www.tomshardware.co.uk/forum/all.html
You also got to remember, as Robert said, age, wear and tear, and of course, a 7 year old technology TV.
Even a modern cheapo would probably be better.
I just did a google and got this from Amazon
Customers who viewed LG 47LM4600 47" Class Cinema 3D 1080P 120HZ LED LCD TV also viewed
4.1 out of 5 stars 4,032
8 used and new from $158.50