It seems like a good deal, as long as nothing major is wrong with it. I'd get a Carfax and have a mechanic look it over, though. $12k is still a good deal of money, and if it's about to require any major service, you should negotiate a lower price. If the frame isn't straight, you might want to pass.
FYI I bought my '07 Limited 4x4 for a little over $20k with 23k miles on the odometer. I still have a month left on the full factory warranty, plus whatever the drivetrain warranty is. I paid nearly twice what you're being offered, but I also have nearly 100k miles less wear and tear on my ST. There are so many other factors; you'll need to decide if it's worth $12k. It never hurts to make a counter offer...