great post:
a little difficult to answer, but, here i go. if you are going to buy and hold the AF 15, then i would say it is a good idea to sell your whole personal collection for the one book in your target grade. it will be alot easier to buy a 200-300 dollar book one at a time until you can rebuild your collection. but, these prices on the AF 15 are only going to go higher and higher. actually, it is how i have managed to purchase my books. i build up a collection. sell it, and buy a high grade key. as i have gotten older, my collection has shrunk, but the quality of my collection has gone up. Now that i am 50--i am liquidating my collection to pay for my kid's college in 10yrs. otherwise, i would do exactly what you are suggesting. sell the whole collection and take that money to buy a super nice higher grade AF15.
Remember, your childhood is yours--no one can take that from you as long as you have the memories. Books are replaceable--but, sometimes finding one in the grade and for the price you want to pay is hard to find. I know i kick myself for not buying some things i passed on in the past. But, the search is fun and finding a book to help finish your collection is a real rush. When you start, just think as though you are starting new and fresh--but, you already own the big score!
cheers and good luck!