Well stores seem to be in the mood
Shiva Store news

Shiva Store :
Shiva Store Link

Will we find some 3DGS store one day ? just laughting it is not some essential feature with 3D engines, people have worked for years without that LOL

Well Perhaps for the next Mobile 3DGS Android support :
Store for low poly assets, code and any other stuff ...

what about DexSoft, Arteria3D, ...?
They are not large but they support gamestudio.

I think also Terminal26 exists out there

Yes, but DexSoft, Arteria if i'm not wrong are also present on their asset stores
I think this a percentage per sell taken by the asset store.
Users are free or not to use the asset store, but it can be good , once you have registered you can buy one or more products from the same applications could it be inetgrated on the editor or in a browser !

One also mustn't forget that there is a huge difference between such a store and a general provider like Dexsoft or 3DFoin. One thing is that there often is a hassle when importing e.g. a model and getting things like animations right (not all shops offer proper files that fit into every engine just out of the box). The other thing which most likely forms the major one is the engine ready content. You cannot buy yourself a particle system, pathfinding algorithm or editor extension anywhere else. Especially when thinking about Unity it's awesome what can be found in their store. I think any engine with a considerable amount of users just has to have such a store nowadays...

probably there are a couple of cool 3dgs plugins, script packages, shader collections, or little tools made by users, which could be sold in such a shop easily, if it would exist, but they remain for private usage, because of the absence of it.
if you created something useful, it is not so easy to sell it online by yourself, without any publicity. e.g. a hobbyist could sell some things for a little money to finance buying legal softwares instead of torrent, and new users could see it is a cool engine with a lot of possibilities.
and from the shop popularity it would be clear to see it is worthy or not to develop and sell anything for 3dgs users.
if you make a plugin or a tool, you make totally different quality if it is a free contribution, or if you sell it even for a few dollars... e.g there are exact requirements for selling anything in unity store.