If it has the matte screen and 8GB RAM, then $800-$850 is what I would pay ... it would have to be immaculate though, and come with the original box. I would be on the fence for any more than that, because don't forget, unless the seller explicitly stated, the battery is most likely NOT new, and that's another ~$100 to replace if it's near the end of its life.
Also, FWIW, late-2011 model is the last 17" that Apple released. So, it can't be a 2012, unless the seller means that's when they personally bought it (i.e.: In 2012, but before the mid-2012 models came out.)
During your thoughts on it, keep in mind that aside from the 17" screen, which isn't available anymore, the rest is 4 year-old tech inside. That fact alone is enough to make me think that even $800 is a bit much to be paying, even for matte and 8GB.
Oh yeah, and if that HDD is original. 4 years of spinning platters will have taken its toll on the lifespan of it. Something else to keep in mind. Expect to have to replace the HDD sooner, rather than later. Superdrive too, for that matter ... and fans! Those are ~$50 each on ifixit ... another $100 to fix sooner rather than later. Not such a good deal after-all, taking all that into consideration. I'd like to change my original number to something lower ... $500-$600.