Talk to your players about this
For any situation where you want to change the rules (either mid-campaign or pre-campaign) you should talk to your players about the rule change and why you want to make it. Ultimately any change to the rules should be making the game more fun for you and your players.
If your players like the proposed rule changes then great, you've gotten everyone on the same page.
You should be prepared, however, for your players to say that your suggested rules changes are not something they will enjoy. If that's the case then listen to them. It may mean you need to change some detail of your plot, or how you award the magic items.
Let's say your players want to keep the status quo with identity: Using your potion shop as an example, you could change Ocean's Potions to be instead:
- each potion comes with a companion potion, which is required for the magic potion to work on someone
- the price of buying a potion is that the character's drink a "wild magic" potion (which rolls on the sorcerer's Wild Magic table for its effects)
- it just sells one copy of a potion
How can you make identification more difficult?
My house rule for this sort of situation (which I established with my players in a Session 0) is the following:
There are three tiers of items:
- Magic Items
- Regular Items
Identify (and other related methods of identification) then has the following properties:
- For all items identify will reveal whether or not it's magical
- For non magical items identify will find out what the item is, its classification (martial/non-martial weapon/other item) and its purpose
- For non-cursed magic items
- a ritual identify will get the current beneficial properties
- a level 1 identify (using the spell slot not the ritual) will get all of the (current and future) properties of the item
- attunement and experimentation will get you some of the properties
- For cursed magic items a level 2 identify will reveal the curse's effects
- For Artifacts
- A level 3 identify is required to get the main beneficial properties
- A level 4 identify is required to get all the properties
- A level 5 identify is required to find out the method of destruction (of which there is only one)
In game this tiering is justified as the magic imbued into the item/artifact resisting the magic from the identify spell and the spellcaster has to overcome that magical potential in order to extract the information from it using identify.
This has worked well in the last campaign I ran. The players were happy with the changes, and how they affected their character's decisions (for example, choosing to attune to an item they could not identify despite the risk that it might be cursed was worth the trade off for one player).
What if I don't want to houserule this?
The DMG has a variant rule for making identification of magic items more difficult in your campaign:
Variant: More Difficult Identification
If you prefer magic items to have a greater mystique, consider removing the ability to identify the properties of a magic item during a short rest, and require the identify spell, experimentation, or both to reveal what a magic item does.