My customers and the shop owners as well always get the icon with a stinky old shoe when they are inside the shop.
I tried several things to get rid of it: moving the sellable frogs outside, then the fridge, where the customers can take things out themselves. I put some plants/flowers inside. The tables are always cleaned at once. The shop owners shower regularly.
Nothing helped!!
Then I thought: maybe I need a ventilation and I put one on the roof. No change.
If I use such a ventilation - do I have to build in in a special way? Or is it enough to just put it somewhere?
I mean: they must have a sense... or are they just decoration?

I'd be very happy if somebody has an idea, why my shop stinks, and what I can do against it.

Are you sure the toilet isn't blocked up?

Assuming it's not, then what the hell are you selling in that store, anyway? And are you making sales regardless, or are potential customers running out gagging?

Do you have:
1. A GTW display/shelf of any kind? Cause these can get dirty and need to be cleaned. And they get dirty SUPER FAST when customers are only browsing. What do they even do while browsing? Spitting?
2. GTW shop signs? Like the neon ones. Because these can get broken which (feel free to correct me if I'm wrong) can lead to customers complaining.

Although I have had days when literally nothing is unclean/broken and yet they're still complaining. Personally in this case I just show them the door.

Toilet is free and clean.
I am selling collectable things like frogs, fossils, crystals, sims-trophys, easter-eggs. And selfmade paintings. Sometimes a little furniture (lamps, chairs).
As I said I put the frogs outside because I thought they were the problem, but they weren't.
I have some GTW-displays/shelves and they get cleaned as soon as I see them get dirty!
Nothing is broken.

The customers and shop owners wave with their hands and say "pooh" or something like that as soon as they enter the shop, and the "stink shoe"-icon shows up above everybody's head.
But they still buy things. I can talk to the quite normal.

It's probably one of the shelves or displays, that's what the shoe means... Sims never let that speech bubble out, unless there is a dirty display.
Check if every single one is ok, ant if it is, and customers are still complaining, then there is a bug in the shop.

After closing the shop in the evening my shop-owners control and clean up everything.
The same before I open again.
I have read about a "cheap fridge smell", that's why I put this special fridge you can buy since GTW outside.
But it still smells inside.
It is a self-built shop.
Is it possible that I did something wrong while building it? But what?

This is only happening to me with one sim. She always thinks the main floor of the building is filthy, while the second floor and the outside are fine. No one else is bothered by it (whatever it is) at all. I did have the snack fridge in there to let my sims eat during their break, but they kept leaving plates everywhere so I just set it for sale too. I've searched the store every time it opens to find the cause of her displeasure, but I can't find anything wrong.

EDIT: Never mind, I'm just an idiot. She has the squeamish trait and I had bugs for sale.

I have this happening in one of the rooms in my house. I've practically torn the room down but cant find the stench. IDK what to do so I ignore it.

Sometimes they place their plates and cups on the bookshelves or the bottom part of tables. I've even seen one put a plate down and it was floating in midair inside of the wall! If I hadn't seen them put it there I never would have found it.

Use Truth or Dare by madonna

I had to sell the register counter and buy a new one to get rid of my smell. It was the only thing that worked.

I have found random dishes scattered around the shop in the past. Plates can blend in with their surroundings a bit too well.

So far I haven't found out what the reason for the bad smell is.
I opened another shop - even on another computer - and short time after opening the customers started thinking of the stinky shoe.
Maybe I'll give it a try with a new counter, perhaps the more expensive one!
And I could try to put the things to sell on normal tables/racks, because they don't get dirty!
Thanks so far for all your ideas.

Make sure you assign an employee to clean the store. If you have one of those three layer displays, they are often hard to tell that they are dirty, but they are.
I have a Sim that has the neat trait. Let them do their thing after the store closes. They will clean the bad areas and have fun while doing it. I also purchased the Fast Cleaner (or whatever it's called), so that speeds up this process. Good luck.