I'll start us off by saying that I really really really want PP restoration items sold in marts (I don't know if that's a thing with the newer games though, I only played up til Gen 5), and I don't get what stopped GF from making it so. If there's a valid reason then I guess I just haven't researched it enough. Would allow them to make more expansive dungeons and not have to worry about the players running out of PP everytime, especially if there's no spots to heal anywhere.
Not only that, but I kinda wish stuff like the Max Revive and Rare Candy were sold as well. I know it's gonna be stomp city if someone can just farm Rare Candies by buying them in bulk from all the Trainer money, but it's as simple as adjusting how much you earn per battle, making the Amulet Coin hard to acquire, and setting the prices for these items fairly high. Would be good for those folks who're just not into having to search every nook and cranny on routes to find hidden items, no? And yes I know that the Revival Herb exists and works exactly like the Max Revive, but I just don't like it lowering friendship... especially since I like Return.
Most games work in terms of sugar - the game leads you to do something - and vinegar - the game leads you away from something.
Not having PP restores is a subtle vinegar - PP is part of how moves are balanced; generally speaking stronger moves will have lower PP and, if strong enough, drawbacks like low accuracy or needing to recharge to prevent people from just spamming them in harder dungeons. They're a resource just like HP and other items, and so, by having few PP restores and not wanting to go back and heal everywhere, you're lead to use other strategies besides "spam Blizzard / Hyper Beam / whatever".
You can farm them via Leppa Berries, but that's another vinegar - spend real life time doing the farming to get the items.
Same thing about Max Revive and Rare Candies, even the Revival Herb has a vinegar in friendship. The game doesn't want you to get Max Revives without any consequences, and you don't want those consequences, so you'd rather use the more expensive combination of Revive and a Full Restore, or if you're in battle, you'll wonder if you really want to use that complete get out of jail card, or if you can leave yourself to attacks before healing the other turn.
These things aren't bugs - they're features. Especially since no other trainer will ever use Revives or PP restoring items (except maybe a Leppa Berry).
I think the only way these could ever be placed in marts and balanced for a playthrough is in a hypothetical challenge mode in which trainers can use these items. This way it feels less like a massive advantage to trainers and the rare candies would be necessary for making some Pokémon viable if you want to have a certain rotation.
I do agree that evolution items - a la Reaper Cloth, Magmarizer and so on - as well as the novelty Pokéballs could be buyable. I know in Gen VIII the bargain shop in Stow-on-side sells the former, although it's a rotating non-deterministic stock, which is ... less than ideal.
Maybe a bazaar place where some days of the week different people will show up to sell their stuff? So we could have a Pokémaniac selling evolution items in say, Wednesdays and every Saturday a Johtonian will show up selling the Pokéballs they got from their relative in Azalea. I don't know.