It's an interesting idea that you just posted here.
I think you mean making a library of graphic designs similar to xatframe, but instead of being public/free and pre-approved you want it to be a kind of store where they can buy them directly. However, each user has different requirements for their designs, for example, use a specific chat name or a color, so this library would not work as a store but as a gallery of designs.
There are currently many external and approved galleries that you can use to post your work, for example: Behance, Flickr and if users like your work they can send you a message to make a request and/or purchase.
Another negative aspect that I see from this suggestion is that it would take a lot of work continuously, that is, xat should delegate people to moderate or approve the content that is being shared in this store to make sure that inappropriate content is not shared or not this using it correctly. Also they would have to find people who have knowledge in some languages like CSS, HTML to be able to examine the code of the players or xatspaces.
And taking into account that the store would belong to xat, if would have an effect on misunderstandings.