That doesn't sound like too incredible of a deal to me. The 7's all drop substantially in price fairly quick. The 2014's are here, so that's a 2 year old car with older tech...... Also, never buy the preowned cars unless you get a great preowned warranty through BMW with it.
Personally, I drove the active hybrid as a rental awhile back and I couldn't stand it at all. It was a horrible experience. It didn't know what it wanted to do. I don't think BMW executed their hybrids properly, whereas Toyota has had their hands in it for a long time. I think if efficiency was important, the Diesel 7 or the smaller 740 engine might be a better value ---- but you must drive it and see whether you like it or not, at the end of the day, you have to live with it
Im not sure 70k is a good price for an active hybrid -- but at 5700 miles, that's very low miles for a 2012, maybe it was a demo or something.
Please make sure you drive it for at least 20 minutes, I really couldn't stand the one I had on loan because of the jerkiness of the drivetrain, I felt like a Prius would have been a better choice for a hybrid.