For grey stamps I am still slightly on the fence about the ink transfer theory even though it makes total sense and is probably most likely scenario
Heres what you can buy: 151 Pokemon Centre Elite Trainer Box Contains 11 booster packs, 2 Snorlax promo cards, various other accessories 151 Elite Trainer Box Contains 9 booster packs, 1 Snorlax promo card, various other accessories 151 Poster Collection Contains 3 booster packs, 3 promo cards, poster containing all 151 Pokemon, card illustrations 151 Binder Collection Contains 4 booster packs, a nine-pocket binder 151 Booster Bundle Contains 6 booster packs 151 Ultra Premium Collection Contains 16 booster packs, hyper raw Mew ex metal card, Mewtwo promo, various other accessories 151 EX Boxes (Alakazam & Zapdos) Contains 4 booster packs, one double rare promo card, two other promos 151 Mini Tin Collection Contains 2 booster packs, one Poke coin, art card matching the tin Here Are Some of The Card Artworks You Can Look Forward To Pulling
The last consideration for double sleeving your cards is accidents
Even more concerning is when you trade for cards in person you'll have to look much more carefully at the cards, especially the highly desirable, more valuable ones