Are they advertising it as an S mint mark? If so, run. As others have stated, that doesn't look like a MM to me, so real but with AT would be my honest assessment.
I'm a Morgan guy and I too am thinking the MM appears auckward and shallow. Look in to it, good luck.
My only real question is.... did the OP buy it under the impression it was an 1883-S? If so, he was mislead.
Even before you examine the mint mark you see a HUGE scratch on the jaw. That may be why the toning was applied in the first place. Trying to change the mint mark is just greedy.