Based on what you have said above, I'd still be inclined against, but I don't know your preferences and collecting goals.
if the book is complete but labeled purple due to trim, is that the worst of it, or does it not present well? Worst case, you've got a complete low-grade copy of the third most desired comic, generally speaking, on the planet. I would think you could get more in cash for it and buy what you want, I'd there's approx. $4k of books in that collection you would rather have.
Also, do you know it was trimmed, or was that identified in the grading process? If so, I would want to know how they are certain since so many say trimming is the toughest resto to detect accurately. There's a pretty broad variance in size among SA books from the same print run; I'm just saying I'd want high confidence with a key of that prominence.
That aside, a good litmus for you -- if you converted your AF 15 purple point-five to cash, is there a book you'd rather have for the $$ than what is in the collection you are considering trading for?
That's my 2 cents; hope it helps you in the way you were looking for. Can you post your book, and/or say more about your target books from the collection and your collecting goals in general?