Mostly because I don't believe items would really add anything to the game- I can't really find a compelling reason to include them. Also most RPG games in my experience lose the money balance very quickly- like a quarter to halfway through the game the player accumulates so much money that they can buy pretty much anything.

Items always have a very important role in RPGs.

1. Items as reward: it is just satisfying to find new and better items after a challenge as reward.

2. Items as progress-key: Items are often used as obfuscated keys to progress to new areas (craft X and use it on Y to receive new monster Z).

3. The collector in us: Many people like to collect items, it is a very strong motivation in RPGs for some people.

4. Items as character progression: Same RPGs just use items to represent the progression of the player character (aka gear based).

5. The businessman in us: Some people like to trade/buy/sell items to gain more money.

Money is just a convenient way to compress large amount of items which are no longer useful for the player, but useful to users (trader).

I'm not sure if your reasons against items are honest. Well, I tended to exclude some aspect in my game, because I wanted to avoid the effort to code and maintain it. In this case I often thought, that it was actual not really neccessary to include this feature, and I looked for some reasons to support this assumption. Eventually it didn't work like this. If you want to leave something out, you should think about why it is really uncessary. The same for adding new things, is it really necessary to include X to support your core game mechanism.

But in your case you take an existing, successful game design and removes some parts, which could break down the whole design.

I guess my real point is that with the Pokemon games specifically, the seeking, capturing and evolving of Pokemon seemed to fulfill all of your points (reward, story-progression, game-progression, collection, character-progression, trading) so well in fact that money and items and stores took such a back-seat that they might as well have not been there at all.

I'm always a big fan of removing as much as possible, so, it seems that you are convinced that items/money are just not necessary to support the core gameplay Then get rid of it and try to get out most of the Pokemon aspect. Eventually you should think about the RPG aspect, do you really need character progression or whatever you think is an RPG element ? Take a look at Zanzarah, a pokemon like game, without any RPG elements.

In the context of a pokemon game, there are only a few items that I think the game would really suffer without; none of these items would necessarily be sold in a store, so I'm not concerned about that part.

Items:

HMs - I actually wish the pokemon games would have made a lot more use of the ability to customize a pokemon's moves, temperament, gender, color, and stats with items. Hardcore pokemon players have to do kind of disgusting things to get a 'perfect' aka fully customized pokemon.

Key Items - The pokedex, bicycle, flute, and such are very important to the plot. It seems very limiting to not be able to give your player progression requirements like "go talk to NPC A and get the KEY to open the gate and be able to progress to the next level".

I think I'll be making an RPG without "items" too. But like your pokemon inspired monsters, it still has collection of things to grow your abilities in interesting ways. I think it helps to take a look at what role items would play, like Ashaman described. If you find that one or two elements of items might be missing, consider adding those elements to your monster collection, unless you want to explicitly leave it out.

Transistor is an action rpg where you collect abilities that can be combined together in interesting ways. It technically has no items, but the ability system is complex enough that additional items aren't really necessary. The interesting set of abilities allow you to grow in ways that kind of take on the role of items, even though there's no trade system.

*Edit: How about taking a look at the role money and stores play? Stores are a way for players to have some control over which items or abilities they get, but there are plenty of interesting ways this can be done without the need for a store. Currency isn't required for trade, but helps smooth out it's rough edges (which you may or may not want to do). Some games use experience points like a currency, allowing you to buy points in different abilities. With options like this, I totally don't think money or stores are necessary. Items/stores/money can definitely allow for a fun trading aspect to the game, but depending on the game's focus and theme, it might be better to cut this stuff out.