I have always wondered about any coin with toning. I've wondered if it is natural of if it was caused by 'outside' sources. I have a friend who works with metal and he asked me if I had a coin. I gave him a Washington quarter. He then grabbed a gas torch, lit it and put the heat to the quarter I had given him. I watched as the heat from the torch turned the finish of the quarter to the, what we all would call, a rainbow tone after just a few minutes. He shut the torch off and said, "there, a toned coin". I was stunned. So that begs the question, "How do you know if the coins you get are natural toned and those that have been possibly manipulated" ?
I don't really like it, but the premium is small for the grade. That's a pretty common date in very high grades. A 2K coin on most days. I would speculate that if it were any other date, it would have never received that grade.
Standing Liberty quarter is really nice design coin. I like toned coin but not this quarter. Too much dark toned eventhough it is MS 67. I wouldn't buy this coin. The Roosie dime is nice tone, but like most have said AU is not where I would pay for that coin as well. If you found it or got it almost at melt then sure.
Seems more like toning breaks than wear to me. I'd call it MS-65 without much argument by the images. For those who say that they wouldn't buy the OPs coin as an AU are really missing the boat. This would be a spectacular pickup at AU prices IMO. Sure the dollar amount isn't too great, but a nice natural toner for a good price? I'd take it.
If by 'badly tarnished' you mean 'attractively toned' then I agree. You need to make an appointment with your eye doctor and get your prescription reevaluated.