It's a good price, but I wouldn't. 6 months is too long, IMO. 1-2 months, OK. Randy has a good point though in that the 55" has been unofficially discontinued, so you likely couldn't find a new one somewhere.
It depends on what you're saving over a new one. If I could get a new one for $100 more I would go new. But if that is the size and TV you have been wanting and you're saving a reasonable amount of money, do it! By the time you see this and get back to the store it may be sold...good deals have a tendency to not last long. Be aware that some stores won't accept open items as returns though, but you'll still have the manufacturer's warranty.
I wouldn't worry about the hours on the TV. Consider it "break in" time. The TV should last for years, but even if you replace it in a few years due to defect or the desire to upgrade, you can do so with a clearer conscious knowing that you saved a lot of money on it. My last TV was a return model (saved $200 over new) that functioned well for me and I passed it on to my brother after a couple of years. He loves it and it will likely last for years to come.