The bags I use for paperback RPGs have an adhesive seal across the flap. I quit using the seal a long time ago, as being a bit of a hassle. They cling onto the book coming and going. It’s a thorough seal, but only good for indefinite storage.
I’ve seen some bags that have the adhesive strip on the bag instead of the flap. That’s easier to use, but still not handy I think.
I began using a single piece of scotch tape as a bag closure. Tape works, but is too adhesive really. My wife will fold a tape piece over partway to create a tab, but I never remember to do that in practice.
Anyhow, I’m trying out 3M Post-It Flags as flap closures. They hold nicely and are easy on/off/on.
The big question will be heat and time. I’m thinking they’ll be ok, as RPG books like this are usually stored boxed or shelved. We’ll see.
EDIT Aug 12 2019:
I’ve quit using the Post-It Flags as bag closures.
In isolation, they work fine. But often modules and booklets are packed tightly together on a shelf or in box. And the friction of pulling out one item can peel off the flag of that item or an adjacent item. In particular, when adding/removing the book vertically from storage.
How has the post it flag experiment been going? I am interested as I too have trouble with tape. Also where do you get your bags that are the right size for things like Avalon Hill bookcase games? I bought a FASA Star Trek rpg box set from you and it came in a bag and I want to get more of those bags.
So far the Post-It Note Flags are serving well. One specific problem I’ve noticed is that they don’t do well when… say… magazines are packed tightly vertical in a box in opposite directions: The post-It notes can get peeled off when you remove or insert a magazine. That’s a real edge case, I know.
As for the bags, I’m not ready yet to sell the larger ones. But if you ask when placing your next order, I’ll add a few to the shipment.