I guess this needs repeating but for any newer collectors looking on. Key date coins cost a bunch of money making them primary targets of the thieves. IE they are counterfeited or altered the most often. And this includes all series of our coins even moderns are cleaned and artificially toned at times. Until you are well educated in authenticating and identifying alterations buy them certified by one of the following firms PCGS,NGC, ANACS, or ICG.
To the OP, I would go back to whom you sold it to. There is no defense for forgeries, and if this person is interested in selling coins at all he would be well served to take this bad fake back. If not, I am sorry to say you have just paid for an expensive lesson.