You have to check for both running hours and people jabbing their fingers into the screen. Those screens scratch easily, I'm sure there are some floor model HDTVs out there with nicks and scratches in the screen from people poking them or running their fingers on them.
$800 including a 3-year warranty is an ok deal, not good, not great - because by the time you are done with taxes, it's $900 and it looks like you can get one new for $1k-ish online and pay no taxes and depending on the vendor, you get free shipping.
Regarding the warranty:
One theory is that if it hasn't died yet, it probably won't, and _any_ degradation of the picture in the next three years would be ground for them to likely replace the set with a new comprable model since it will be discontinued and most equipment doesn't get repaired anymore.
Another theory is buy new at the going rate of $1k online and foret the warranty. If the set dies in a few years (unlikely for a Sony without external causes like a lightning strike on the cable lines or power line problems), it'll be outdated anyway.