I purchased an answer key for the first edition of Mastronarde's Introduction to Attic Greek. This key is from the old, out-of-print edition of the book. The answer key for the first edition is also out of print, and prices are rising on extant copies. I paid more for the key than the textbook.
If anyone else is going through this book, leave a note and I can (eventually) scan the thing--give me a week or so after the initial request. I have no real qualms posting a key that Mastronarde says on his website will not be printed again. Please note, there are errors in the answer key (as well as the textbook)--those found before the publication of the second edition are available on Mastronarde's site as PDFs; I will either add to the scan as errata, or will link separately.
If you want this key and much time has gone by since my initial post, you may have better luck PMing me (esp. if I have not been active on forum--years have gone by in the past).