i typically START prices at 50% retail, and go from there. if you can find a the tv selling for 700, 350 is the most i'd even think of paying for one used.
ppl don't know how you treated the tv, how used it is, and there's no returns. all these thins negative affect a tv's price.
all that being said, this is why i've never sold a tv. i use a 'rotation' instead. new tv goes in the theatre, tv that was in the theatre moves into another room that could use it, and then the tv that replaced can move, etc, etc, until the crappiest tv in the house gets tossed. for example, the reason i bought my new f8500 this year was because the CRT RPTV in the living room finally went. moved the old plasma from my theatre into the living room, f8500 in the theatre, and the rptv to the recyclers.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One