I made a quick count, but I think it's even less than 5% (7 out of 224). And as I mentioned earlier, some of the other books are borderline books like New Mutants #1-20.
I wasn't the one hitting the button and I mean no particular offense here, but you have a ton of Copper books in your sales thread - far more than just 7.
All the New Mutants books, Thor #344, Captain America Annual #8, Guardians of the Galaxy #1, ASM 289, ASM 296, Iron Man #282, ASM 245, ASM 256, ASM 261, ASM 263, ASM 268, UXM 244, etc, etc.
Your thread is pretty much the perfect example of a Mixed Age thread :shrug:
I'll easily agree with you on the Spideys, Iron Man #282, Guardians of the Galaxy, X-Men #244 and the Cap Annual, but not so much on the early New Mutants and Thor #344. They're 1984 or before and more like "tweener" books to me.
I'm sure you can point out other books too. But the vast majority of listings in thread are from 1984 or before.
If you use 1984 as the cut-off, and I think that's fair, and my count is more careful now, there are 25 post-1984 listings out of 224.
If you're a glass half-empty kind of guy, that's right around 11% copper.
Having these books in GSB has never been an issue before. In fact, I've had these books in GSB before and it's never been a problem. Until now.