Possibilities to pay with different currencies.
For example Gaya/Won/What Ever.
If you could expand that as an option, that would be really cool.
Another wish would be to store unsold items safely.
I think most players have already had enough negative experiences.
It's also stupid for an admin to look at the logs every time to see what might have happened. Certainly a problem that does not exclude self-infliction. But if we are talking about a perfect offline shop, a user-friendly option would certainly be very good.
For example, an area for the player to list what was bought for how much at what time, what was not bought would be useful. That way you have a list. With clock times. And the player has feedback, the team can work with it.
Above all you would have another nice feature.
What was shown above me worked on the server I played on only up to a certain time. At some point nothing was logged anymore. So, useless as fck.
I'm afraid I have no idea if this still fits the theme.
But Aeldra, for example, has such a system: an average value is calculated for an item above a certain quantity. From this, the minimum and maximum value of the item is determined.
So that fake items or trolls have no chance at all. And the market remains reasonably stable. This way, a server can also act on its own to solve possible market problems.
Optionally, this could only be applied to uppitems, consumables and other items, EQ with bonuses is always weighted differently.
I don't think an offline shop needs much more than what has already been suggested.
I would put my main focus in addition to a suitable shop search system. Because the combo is really well appreciated.
As you may know, most shop search functions of various servers are designed for their own product.
Maybe you could take action there as well.
But whatever you do, remember that metin players are not the brightest.
So user-friendliness is the first priority.
This would also set you apart from people like dracaryS and co.
For the product, I would recommend a standard version, which can possibly be used by every server, no matter what concept etc.
And then additional functions that could be integrated with your help as an additional service. E.g. the shop search.