Is it possible to implement a "sold out" notifier?

Would be most useful to my humble opinion.

I believe this is done now?

Don't think so at least if we assume that they mean offline notifications. ChestShop currently only notifies when online.

I guess this issue can be closed as it is not possible and quite ancient

Well it's not really done, if you could specify what you actually meant by your original text as it's not really that clear what you would expect to happen in CSN then maybe it could actually get implemented.

Oh jeez, It's been more than 2 years ago.
If I remember correctly, it was something like this:
A player is offline for several days. Players buy stuff while the chest owner is offline and the chest becomes empty.
So after a few days, the chest owner comes online. He sees that he has a lot of sales. Checks the message.
Message says e.g. you sold 200 items, 10 this, 20 that, and so on. But chest owner has no clue how many there where in each chest. Is it sold out? Are there still items in it? So he goes to his shop, and needs to check and fill every chest.

If the notification would contain something like:
PlayerABC123 bought 50xstone xx hours ago (+€ 50) and is sold out
Or:

PlayerABC123 bought 50xstone xx hours ago (+€ 50)
Chest is empty now

then the chest owner immediately knows he has to act on this or we will not sell anything of that item. Or he can remove that chest and sell other items on that spot.