Shut off at night, oldest part is probably the ram, not sure on how many years but next to that is my first card which is almost a year old and everything else was bought during November of 2012, the second card was bought less than 5 months ago.
I was going to say between $850 and $1000, depending on the buyer. It's a rather competent computer, but most people are incredibly distrustful of used computers, for good reason.
I'd say around $1200 give or take a few hundred. Looks like a solid system. I'd recommend you give Ebay a shot. Who knows, you may get more for it. You could also try parting out your system. Usually you can sell your computer faster and get more money if you part it out.
Not noticably unless you overvolt by more than 10%. He's pretty safe with only 4.6 GHz on an FX 8350. Though being used in any form tends to dramatically reduce the expected cost of resale.
OCing does not make it much shorter life its like saying its only going to last 10years instead of 15 like it makes a difference like you are going to keep it that long any ways.